Human Intelligence Engineering

Hi, I'm Bernhard, a Software Architect, Writer & Teacher. As "AI-powered robots are taking over", I am convinced that the ability to continuously learn and apply human creativity to technology is one of the most valuable skills to develop now.

I also write on Substack at

Read more about my background and what to expect from this blog here.

Thinking In Lists

Have you ever tried playing chess against yourself? Unless you are a chess purist or have a split personality, it is not much fun.

The pleasure of engaging in a game like chess comes from challenging another mind (human or otherwise). From not knowing what the other is planning and the uncertainty that comes with it. When two opponents share the same brain, there are no secrets or surprises.

Another player makes the game “real.” When you have found a weakness in the opposing position and hatch a plan to exploit it, your opponent will prove your plot right or wrong. The stronger the rival, the harder your ideas will be tested. Left alone, that “clash with reality” can’t occur. You can try hard to emulate both separate sides, but as the two are coming from the same mind, all you do is evaluate yourself, which has limited value.

We are in a similar predicament every time we are trying to understand something or come up with an explanation. Because we are the creator of our thoughts as well as their recipient, we are biting our own tails whenever we try to figure out if what we think is correct or not.

To overcome the lack of outside perspective, we intend to look at a problem from all angles. The trouble is that not only is seeing the forest from the trees difficult, but that thinking itself is flawed. The mind’s default operational mode is not rationality, and it has no incentive to spot inconsistencies. Instead, it happily paints over blind spots, leaving us unaware of our imperfections.

Why is this so? Wouldn’t it be beneficial to our species to ensure that our thinking is consistent and flawless? The short answer is no. The mind is not designed to think. 1 From an evolutionary point of view, it is more beneficial for our survival to automate thinking as much as possible. Active, hard pondering of problems consumes valuable energy that would be better spent on spotting danger or other life-preserving activities.

The brain is arguably the most wonderful thing in the universe (as far as we know, that is). It can do marvelous things like seeing, hearing, memorizing, control bodily functions, all at the same time and while giving birth to consciousness. But when it comes to learning and rational reasoning, the organ inside our skull is a broken tool, like a water bucket with holes.

Without a clear picture of what we actually think (rather than what we believe to think), efficient learning is difficult. It is essential to shine a bright light on the gaps in our reasoning and bring them out into the open. Learning is as much about gaining new insights as it is about getting rid of false beliefs.

What can we do?

Ideally, you’d have a few like-minded people at your disposal interested in your stuff who could challenge and inspire you (could this be an AI in the future? That would be tremendous).

Lacking such a sophisticated support group, the next best thing to do is to break the mind’s self-evaluation-loop by putting distance between the moment we generate an idea and the time we evaluate it.

Humanity has invented a tool for exactly that problem - pen and paper. Writing helps us capture our fuzzy thoughts and make them explicit and observable. The problem is, it can be a laborious word-wrangling exercise to express ourselves. Luckily, for our purposes we can skip most of the arduous parts of translating our intentions into sentences and simply transpose our raw thoughts onto paper. This can be done much easier and faster by making a list.

The difference between a list and “normal” writing is the intended audience. Usually, we write for others, which makes it so hard because the reader does not know anything about us. We need to give as much context and be exact as possible to avoid ambiguity. The list, on the other hand, is only meant to be read by the author. No need to explain where we are coming from, we can be as esoteric in our descriptions as we like.

There is one non-negotiable requirement, though: Everything on the list has to be written using our very own words. This is crucial to make the list fulfill its purpose.

Looking up answers and copying-pasting them into our list does not help at all - rather the opposite, it only strengthens the gaps in our understanding because it creates the illusion of knowledge. We may write down correct facts, but they are not coming from us.

This is one reason why highlighting passages in a book does very little to increase comprehension of a text. It looks like learning because the passages are important, but they are not our words, they are expressions of someone else’s thinking (underlining text passages can still be a useful activity, but the highlights are merely bookmarks, helping navigate a large amount of information).

Definitions, buzzwords, or concepts we don’t fully grasp don’t belong on the list. Unless we write down precisely this - that we don’t understand those terms (e.g., “Technology X is a distributed-self-contained management system - what the heck does that mean?”)

We list what we know (or believe to know) and what we don’t know or understand. This is a fast and fluent exercise. It does not require much thinking. Then, very soon, we will have exhausted what’s on our mind.

I love making lists. Whenever I am stuck, I start listing my thoughts. This post here started as a list (as all others have) that I then successively transformed into full sentences. The moment I got stuck, I switched back into list mode, and so forth. I’m always shifting modes. From slow and exact writing, building up a single line of argument, to the immediate and quick jotting down of multiple strands of thoughts (there are other modes, but that’s a subject for another post).

When I begin a new task, I start with a list. To clarify my intentions and beliefs and to prepare myself for the work ahead by bringing into focus the things I’m sure about and those I’m not. This way, I prime my mind to find answers for the gaps I have and to look for confirmation of what I assume to be true.

Lists are helpful for any kind of mental task. They expand our short-term memory. They force us to think sequentially and turn vagueness into explicitness. And list-making is a fun activity, even relaxing, similar to meditation and its benefits, where we step back from being mindlessly captured by thoughts and instead gain control through observing our mental states.

Video-Learn confidently with mental models

Knowledge as design - does knowledge have a purpose?

What do we value most in life? Health is an obvious choice, next to love and meaningful friendships. Money is certainly on the list too, but controversial, at least when pursued narrowly at the cost of everything else (though few would dispute that a good life depends on owning a sufficient amount of it).

Uncontroversial is the value of another item: knowledge. Our post-industrial society increasingly depends on the creation and transfer of know-how and technical expertise. We usually consider knowledgeable people as intelligent, which often correlates with higher social status and financial reward.

But what is knowledge? It is surprisingly hard to define. Does memorization of a lot of facts (e.g historical dates) count as knowledge? Then your internet-connected computer would be the most knowledgeable entity ever created. Facts are data points. Taken by themselves, they have limited value if we can’t explain their existence and are unable to predict future facts. In other words, we lack understanding.

Furthermore, the value of knowledge depends on its usefulness to solve a problem. In particular, to solve my problem. Ask “How much do you know about X?” and I would reply “to accomplish what?” (which is nicer than an engineer’s favorite retort of “it depends…”). I know that a combustion engine needs oil and gas to function and have a dim idea about gears. Enough knowledge for me to drive a car, understand the meaning of warning lights and abstain from shifting into reverse while the car is moving forward. If you ask me, I know all there is about cars. However, if tomorrow I wanted to start a new career as an auto mechanic, I would immediately find out that I know very little.

Figuring out the purpose of knowledge affects how we think about it and fundamentally changes how we teach and learn. By asking about the reason for knowledge we move from passive consumption of facts to an active inquiry about the why, how, how well, and so on.

How purpose and knowledge relate to each is the subject of the fascinating book called “Knowledge as Design” by D.N. Perkins.

As the title suggests, the author considers knowledge not something that is out there, given to be consumed without critical analysis (he calls this “truth mongering”) but something that has a purpose, or, in a broader sense, a design.

The first thing to note is that the word design may be misleading. If you, like me, haven’t thought much about design before you may equate it with style (fashion, brands, websites) or a blueprint to build something (design of a house). The author takes a wider look and defines design more accurately:

Design is the human endeavor of shaping objects to purposes

Almost everything is a design. A knife is an object adapted to the purpose of cutting things. Academic knowledge is also designed - Newton’s laws are a mathematical tool to explain the motion of bodies. So are processes, claims, and even historic dates as a part of a larger story (for example, the year 1492 marks a milestone in western civilization). There are only a few things that are not designs (like natural phenomena).

So how does thinking about design help us with acquiring knowledge? Knowing the purpose of a design is crucial, but purpose alone only explains the reason a design exists, it does not say anything about how it solves a problem.

The four design questions

The author gives us a tool to examine designs by formulating four design questions that make up the heart of the book.

The questions to ask about a design are the following:

  1. What is its purpose?
  2. What is its structure?
  3. What are the model cases of it?
  4. What are the arguments for and against its usefulness?

“Purpose” we have already discussed. The structure of a design deals with the parts, components, materials, relations, and so forth of the object in question. A knife has a cutting edge attached to a handle.

Model cases are examples of the object or knowledge (imagine photos of different types of knives). Model cases are in some sense mental models, but in a much narrower way (I come back to this later).

Arguments, the last of the four questions, evaluate a design. How well does it do its job? Given the purpose, is the design well-thought-out or could it be better? While the former three questions could be answered without much context, here we depend on our subjective understanding of the object at hand. I can’t say anything about the effectiveness of a design if I don’t understand what it does or how it differs from other solutions.

The book discusses at length how the four questions can be applied to practically everything. It turns out that looking at the world through design glasses is tremendously useful. Be it how to write an essay, how to come up with new insights, or how to improve learning and teaching in general.

Since knowledge plays such an important role in our life, it is worth looking at how this new way of thinking about knowledge (as design) compares to our default way (as information).

Knowledge as information Knowledge as design
passive active
something to store and retrieve something to apply
out there to memorize result of human inquiry
no purpose born out of purpose
impersonal personal

We shift from the left to the right, from the passive to the active, by connecting what we learn to each of the four design questions.

So how does all this help us with our mental models? Is one better than the other?

How do software mental models (SMM) compare with the four design questions (4DQ)?

The short answer is that 4DQs are a tool to evaluate the subject at hand that leads to a conclusion, while SMMs are a process that keeps evolving. There is overlap between the two but also significant differences. Let’s have a look at each of the four questions.

Model cases

We start with likely the most confusing part. Aren’t mental models the same as model cases? Not really. Mental models, at least the way I use them, are internal representations of reality, that can differ significantly from the actual object. Model cases are examples (of object types, procedures, formulas, etc), whereas mental models are simplifications that are (hopefully) so useful that they can explain complex processes with a few elegant analogies. Coming back to the simple example of a knife, model cases would present all the different types of knives (we could, for example, present their evolution from the stone age until today), whereas the mental model of a knife depends on what goal I have. In my case, I mostly think about knives in the context of a kitchen, so I have mental models about how the sharp side of a blade works and what knife to use for certain materials (e.g. bread versus meat). A chef has vastly more complex internal representations of knives. She might think about angles, handles, all kinds of materials, cutting techniques, and so on.


The structure of a design describes the elements and parts of the object or knowledge. Mental models include structure, too, but only those parts that are useful for achieving the goal. When learning something new, the structure of a model will be similar to a structure of a design. The more advanced the models get, the more abstract they become until they may have nothing in common anymore with the actual object.


The purpose of a design is similar to what I consider the goal of a mental model. However, in the case of SMMs, the goal is deeply personal, whereas the purpose of a design is meant to be independent of the learner.

The goal of a mental model gives clarity about what is important and what I can leave out. The goal evolves. In a design, the purpose is fixed - the reason the design exist stems from the purpose.

In reality, the purpose of a design is still to some degree subjective, because it depends on the knowledge of the learner. A physics beginner in school will come up with a different, simpler purpose for Newton’s laws than a university graduate.


Arguably, arguments are the most important of the four questions because they require the learner to conclude based entirely on their understanding and experience. Purpose, structure, and model cases lay the foundation that arguments are built upon.

SMMs don’t have arguments at all. If they are so important, how can this be?

The reason can be found in the fact that SMMs are a process. Arguments and purpose are two sides of a coin. The argument describes how well a design accomplishes its purpose with the outcome of an evaluation. SMMs don’t have a final step. Instead, we evaluate models during the test phase, where we figure out where the models are inaccurate. Instead of giving a judgment, we correct the model and make it better during the refinement phase (or replace it with a better model if necessary). The argument is ingrained in the process.

In summary, we could say that SMMs are the agile version of 4DQ.

Knowledge as design Software Mental Models
Purpose Goal of the mental model is personal and depends on the learner’s experience and aim.
Structure The structure of SMMs may be similar to the real object but advanced models may not reflect reality at all.
Model cases SMMs go beyond mere examples of designs. They can include model cases but don’t need to.
Arguments The evaluation of a mental model happens during the test & refinement phase, always adapted to the goal of the learner.

The 4DQs are a very useful tool to structure thinking, learning, and writing. The book can be repetitive at times as it goes through the many possible applications of the theory, but once the main ideas are understood it is easy to skip content and focus on the parts that interest you. They are certainly useful for developing SMMs.

Can you spot the 4DQ in this post?

Writing an essay is among the use cases of the theory. Can you spot where in this post I have used the four questions?


  • The first half of this post explains the purpose and structure of the 4DQs. First I made claims about the importance of knowledge, and in particular, the value of looking at the purpose of knowledge
  • Then I described the structure of the 4DQs, describing each question.
  • I finished the first half by completing my description of the purpose of the 4DQ by demonstrating the value of moving from seeing knowledge as information to considering knowledge as design.
  • I use model cases of 4DQs throughout the post, most of the time using the example of a knife. And, to go fully recursive, this answer is another model case of the 4DQs.
  • In the second half I give arguments by comparing in detail the 4DQs with SMMs, with the conclusion that while there’s overlap between the two ideas, SMMs fundamentally differ from 4DQs by being an ongoing process instead of a final evaluation of an object or piece of knowledge.

The Mental Model Of Docker Container Shipping

Programming is a marvellous activity. By typing on a keyboard and feeding code to a computer, a programmer can create entirely new (virtual) worlds out of nothing. There are few constraints on what a program could be - the only limitations are memory and a CPU’s processing power. Whatever you can think of, you could, in theory, bring into existence on a computer.

But since virtual worlds are created in the mind of a developer and not based on physical reality, it can be difficult to talk about what they are or how they work. There is no immediate language to refer to. To overcome this lack of terminology, engineers often use analogies from the real world to describe what software does.

Docker is no exception. At its centre stands the “container”, the main building block of what Docker calls a “standard of shipping software”. One can’t help but see container ships towing heavy loads over oceans. Even the logo depicts the main idea:

Containers and shipping software

When we learn about new technology with strong ties to another technology, we immediately apply our existing knowledge of the substitute technology to fill gaps in our understanding. This “replacement reasoning” can be helpful or misleading, depending on how accurate the analogy turns out to be. Therefore, to avoid misconceptions, it is beneficial to evaluate existing mental models first, as they are the ground on which we build our individual models.

In the case of Docker, we want to know how helpful the idea of “container shipping” is and how far we can take the analogy.

It turns out that there are many parallels between real and virtual containers and that the properties of container shipping cover quite a lot of concepts of Docker.

Containers (those made from steel)

Shipping containers revolutionized international trade by making it possible to move freight between different modes of transport without the need to re-package goods. Due to standardization and a complex system of docking ports, the cost and time it takes to ship goods worldwide have been significantly reduced.

What makes containers so valuable is that they can be stacked neatly on top of each other, thanks to their standardized dimensions. They can also carry nearly any kind of goods and can be used interchangeably.

Docker Containers

Like shipping containers, Docker containers also increased efficiency. Most applications run on Webservers. In the past, it was not possible to execute more than one application per server. Not only was managing a fleet of invidual servers difficult, it also lead to a waste of resources, as applications can vary significantly in their requirements that change over time (servers needed to keep a capacity reserve).

Then a company called VMWare came along and invented the virtual machine. I mentioned in the intro that computers create virtual worlds. They can also create worlds inside worlds. A virtual machine is an emulation of hardware. Each emulated hardware is a new virtual computer (creating computers inside a computer), making it possible to run multiple applications side by side on the same server.

One OS per VM (Source: “Docker Deep Dive” 1)

This makes deploying applications much more efficient. Applications run inside their own virtual machine (VM), and hardware resources can be individually distributed to meet the requirements of each application.

But VMs are big. Virtualizing the hardware requires each virtual machine to include an entire operating system.

Docker takes a different approach. Instead of virtualizing the hardware, it aims higher and abstracts away the operating system. Docker containers share the same OS resources, making them more lightweight 2.

Docker containers are smaller (Source: Docker Deep Dive1)

We talked about why Docker exists, but we still haven’t said what a container actually is.

How do Docker containers compare to physical containers?

The physical container is an enclosing made of steel that carries goods. Similarly, a Docker container provides applications with an isolated environment (the virtual equivalent of an enclosing).

Actual containers are filled with goods. A Docker container runs an application. To understand how the application ends up on a Docker container (and how that is done efficiently), we need to understand three basic concepts:

  • Docker engine
  • Docker image
  • Containerizing an app

Docker Engine

The word “Docker” can mean two things.

First, there is Docker, the company. They develop the tools and libraries required to run Docker and maintain the Docker hub, a central repository.

Then there is Docker, the technology. There are three main components3:

  • The runtime provides low-level libraries and components to run containers.
  • The engine (or Daemon) provides higher-level functionality, most of all the interface for Docker commands.
  • A Client is an application that talks to the Docker Daemon (e.g. the docker or docker-compose commands)

When someone learns Docker, they will spend most of their time understanding the interactions of a Docker client with the Docker daemon.

Actual containers are made out of steel. Similarly, the Docker engine + runtime are “the fabric of Docker containers”.

Docker image

The Docker glossary describes containers as a “runtime instance of an image” 4. An image is a collection of files that are executed when a container runs. They are, we could say, the “content” of a container - in other words, the application.

Images are constructed in layers, where each layer corresponds to a set of files of a container. More precisely, each layer is a set of file changes. When an image is built, layers are executed sequentially, possibly overriding files of a previous stage.

Image layers, overriding changes (source: “Docker Deep Dive” 1)

Layers can be cached. That makes building images efficient because every time an image changes, we only need to update those layers that have changed.

Containerizing an application

The final piece of the Docker puzzle is the so-called process of “containerizing an application”. Or in other words, how do we turn an application into an image that we can then execute on a container?

This is done via a Docker file. In the container shipping model analogy, a Docker file is similar to a list of items that describe the content of a container. Except that it not only lists the files but also describes the order of their occurrence or modification.

Side-by-side comparison of container shipping and Docker

Finally, we get to the fun part! How far can we take the analogy of container shipping to create a mental model of Docker?

Container shipping is straightforward to visualize - starting with containers loaded with goods to the cranes lifting them onto ships and so forth until they reach their destination. For every step in the journey of a container, we can find a similar action in the Docker world. Let’s compare them side-by-side.

Concept Container Shipping Docker
What is a container? Vessel for goods Vessel for applications
What are containers made of? Out of steel that is strong enough to keep contents stable and safe but not so heavy that containers can’t be moved Docker containers are made possible through the Docker runtime and engine, which create isolated environments for applications that can run side-by-side on the same hardware.
Why are containers interchangeable? They have the same size and material qualities Images always create the exact same application regardless of how often or when they are created, executed, stopped or removed
What makes containers efficient? Containers can be neatly stacked on top of each other and moved quickly through standardized ports and between vehicles. Containers are more lightweight than VMs because they share the same OS resources. They allow running multiple apps on the same hardware, each container optimized to the application’s requirements.
What are the items of a container? Arbitrary number of goods File system changes that make up the containerized application
What is a single item? A single good A single file system change represented by an image layer
What is the description of the container content? Container receipt Dockerfile
What vehicles transport containers? Ship, train, truck Hardware + OS + Docker engine
How are contents prepared? The container is filled with goods The image is built
How can filling a container be optimized? Goods are produced near a port The image is uploaded to a repository
How are containers prepared for shipping? A crane moves the container onto a ship/train/truck An image is downloaded, and the container is created
How does the transport start? The ship/train/truck is started The container is started
How is the actual shipping performed? The ship/train/truck carries the container The application runs in a container
How are containers unloaded from transport? A crane is lifting the container from a ship/train/truck A container is stopped
How are the items unloaded? Goods are moved out of a container The container is destroyed
How are containers disposed of? A container is scrapped The image is removed

We can even create a little diagram that compares both worlds:

  • Think of a Docker container as a vessel for applications.
  • The Docker engine (and runtime) provides the enclosing for goods, similar to steel used to fabricate a physical container.
  • The Dockerfile is like a description of the contents of a container.
  • The image provides the content of a container, each file (change) is like a single good.
  • Building and uploading an image prepares the content of a container.
  • Downloading an image and creating a container is like a crane lifting a container onto a ship.
  • Running the container can be compared to a ship transporting containers over the ocean.
  • Stopping a container, destroying it and deleting an image is similar to a crane lifting a container off a ship, taking out the goods and scraping the empty container.

  1. Docker Deep Dive by Nigel Poulton - Safari Bookshelf  2 3

  2. The disadvantage of Docker is that all applications need to run on the same OS - but since Linux is the de-facto standard for most applications, this is usually not a problem. 

  3. A fourth component, swarm, is often mentioned as well, which coordinates multiple instances of containers. 

  4. Docker Glossary - a container “is a runtime instance of an image” 

First Git Mental Model

We make sense of the world through models we create in our minds. How well we understand reality depends on the accuracy of those models. However, we are often not aware that our internal representation can be inaccurate or, worse, we may not even realise that we use mental models at all. By bringing our thought processes into the open, we can shed light on the gaps in our understanding and learn more efficiently.

I have written about the role of mental models in software engineering. I recommend reading that first if you haven’t done so, as it explains the main idea. In this post (and more to follow), I take an existing tool, framework or subject and look at it through the lenses of mental models.

Today’s example is Git. It assumes little or no prior knowledge. If you are already well-versed in Git, I encourage you to keep reading nonetheless. The goal of this series is as much teaching a specific subject as it is about exploring how to construct good mental models in general. There is no shortcut to achieving accurate understanding. As good as an existing mental model may be, we still have to absorb and make it our own; we can’t just copy and paste it into our brains (unfortunately).

What is Git?

The official definition says this:

Git is a distributed version control system.

Does reading this sentence evoke any mental images for you? In my case, the word “distributed” brings to mind several computers running in distant locations. “Version control system” lets me see software that stores copies of documents in an orderly fashion.

This is rather vague. A better way to understand something is not to ask “what” it is, but “why” does it exist - what is its purpose?

What is the purpose of Git?

There are primarily two problems Git is trying to solve:

  1. How can multiple people work in parallel on the same documents without stepping on each other’s toes?
  2. How can we provide a safe environment for making changes without worrying about losing previous work?

2) is part of the solution for 1). However, it is also possible to only take advantage of 2) while ignoring 1) (if you use Git solely as a local version tracker, which is a legitimate use case).

1 - How can multiple people work on the same documents at the same time?

Let’s think about 1) - there is a group of people who want to work on the same set of documents simultaneously. How can they do that? Pose that question to yourself, and your brain will immediately start generating possible answers.

Depending on how much experience you have with a given problem, the solution you come up with can be complex or straightforward. A natural way to find a solution to a problem is to search in our memory for a similar situation that we can apply to the new challenge. How do people typically collaborate?

I see an office where several people are sitting around a table, with paper documents spread out in front of them. They pass files around, discuss changes, use pencil and eraser (or scissors and tape) to make changes.

Something like this:

Although this model describes collaboration, the participants can’t work in parallel, as they need to take turns to change a document. Let’s refine our model by having everyone work on copies instead of the original documents. To reflect that Git is a distributed system, we also let each worker have their own table.

Our new model looks like this:

How would that work? The table in the middle (let’s call it the “origin” table) contains the original documents. If I want to work on them, I first make copies of the documents and then grab one of the tables and go about making changes as I please. Then comes the moment to update the originals. I go back to the origin table and replace the sources with my edited copies.

There’s just one problem: what if someone else has already changed the originals? If I swap the documents, their changes will get lost.

So instead of swapping out entire documents, I need to “merge” my changes with whatever is on the origin table. This works most of the time, except when my change and someone else’s change are in “conflict”. For example, in a design document, someone changed the color of a button from red to green, but I changed it from red to yellow. Who’s change should persist?

A conflict:

I want to keep my version, but what if I change my mind later (because it turns out my colleague was right)? That is the second problem Git solves: as long as we track changes and can revert back to a previous state, we are safe to make changes.

How does Git make that possible?

Assume I have no spontaneous idea. We are then at a typical state during the learning process. We have exhausted fleshing out our mental model (or don’t want to speculate anymore), and we encountered a few areas where we can’t satisfyingly figure out how things work in reality.

This is a good moment to do some research and consult a book or a manual.

Sidenote: why start with the mental model?

You may ask why we didn’t start reading the book before building our model. Aren’t we wasting our time trying to come up with an explanation of how things work without having done research first? I disagree. By having made our current knowledge visible, the subsequent research becomes more targeted. We can locate gaps more quickly, know what kind of information we need or don’t need, and have a map to place new information.

2 - Keeping a log of changes

So how does Git keep a log of changes?

I use Pro Git as reference documentation. A good chapter to start is What is Git?, you may want to read it now.

The main point from that chapter leads us to the answer to our question:

Git maintains snapshots of changes, not diffs

In other words, every change in Git is stored as a snapshot of all files. Note that the word “snapshot” is another mental construct. Like a camera that catches a scene at a specific moment, a snapshot in computing is a copy of an entire state of a system at a certain time. We can use that information to extend our mental model.

Besides working on our copies of the original documents, we create additional copies of the copies and put them in boxes. Each box reflects a document change. To keep a log of changes, each box links to the previous box.

It looks like this:

Those boxes are uniquely labelled. As you may have read in the book, Git creates so-called hashes of documents (you’ll find hashes everywhere in Git). A hash is a string of symbols that is unique for any possible state of documents.

Sidenote: focus on information to fill the most important gaps, ignore the rest

You may want to dig deeper to understand how hashing works under the hood. Or not? That is another common stage of any learning process. The question is this: should I figure out the inner workings of hashes, or should I just accept that they work in some way and move on?

Your mental model can guide you here as well. At this point, knowing how hashes work won’t help you with your understanding of Git. It won’t increase the accuracy of your model at this stage (being a beginner). It is the wrong rabbit hole to go down. Instead, for the sake of simplicity, we accept this gap in our understanding.

Good mental models strive for simplicity by conveying just enough details that are required to reliably predict the behaviour of the thing you are investigating. Any additional information will slow down progress (more about this at the end).

Log a change - but when?

Back to Git. Another question remains: when do we take a snapshot? Could we store every change? That would create an insurmountable amount of data and not be feasible.

Instead, what seems practical is that the author decides when to store a snapshot. Only they know when changes have reached a meaningful state that should be preserved.

How does Git make that possible?

The same Pro Git chapter mentioned above answers that question. The relevant paragraph is called “the three states”.

Three states

A file in a Git repository can have three different states: it is either “untracked”, “staged”, or “committed”. Untracked files (or modifications) are not recorded yet. When files are “staged”, that’s the moment when we intend to take a snapshot (but we haven’t registered the change yet). Committing files is the final step of storing the snapshot.

The three states:

Coming back to the question of when to take a snapshot, the answer is that it is up to the user to make that decision.

By the way, Git introduces three states, although two would have been sufficient (e.g. committed - or not committed). The intermediate staging step makes preparing a snapshot easier, as we can add or remove files and change our minds before creating the record.

Testing our model

We have developed a set of mental models that cover the basics of Git. It is time to test the usefulness of those models by asking ourselves if they can sufficiently explain how things work for our purposes.


What is a branch? The word is already evoking a mental model of a real tree, with its main trunk and branches diverging into new directions.

You can read in detail about Git branches in Pro Git, but we can already explain how branches work in principle. As discussed above, “how Git creates a log of changes”, a branch is nothing other than a series of snapshots that is different from the main development flow. Internally, Git uses pointers to make that work, but all we need to understand now is that changes are stored as snapshots and that each snapshot has a unique label (hash) attached to it. If you want to know if two branches are the same, all you have to do is compare the hashes.

Git commands

We test our model by looking at basic Git commands that you will likely have encountered when you first learned about Git.

git clone
git add
git commit 
git push
git pull

If you are new to Git, I recommend running git COMMAND --help and start reading through the first paragraphs to see if the explanation fits our mental models.

git clone --help says

Clones a repository into a newly-created directory, creates remote-tracking branches for each branch in the cloned repository (visible using git branch –remotes), and creates and checks out an initial branch that is forked from the cloned repository’s currently active branch.

We may not fully understand every detail, but the overall idea fits our distributed tables model of making copies and branches.

git add --help

This command updates the index using the current content found in the working tree, to prepare the content staged for the next commit.

We don’t know yet what an index is but can explain most of what is said here with the three stages model. And we learn that git add moves changes from “untracked” to “staged”.

git commit --help

Create a new commit containing the current contents of the index and the given log message describing the changes.

Same here, and now we can conclude that the “index” tracks changes that have been “staged”.

git push --help

Updates remote refs using local refs, while sending objects necessary to complete the given refs.

We don’t know what a ref is but “sending objects” fits our model where we bring our documents to the “origin” table and merge our changes.

  • git pull --help

Incorporates changes from a remote repository into the current branch. In its default mode, git pull is shorthand for Git fetch followed by Git merge FETCH_HEAD.

This does not fit our mental model. So far, we said that when we are ready with our work, we bring our changes to the “origin” table. But in reality, we “pull” changes down to us. We found a gap in our model. Coming back to Pro git - What is git? we can now understand what is meant by Nearly Every Operation Is Local. If Git was new to you and you followed along with the steps of this article, I bet you have now a stronger understanding of what “everything is local” means compared to when you first time read about it.

“Real-life” testing

The ultimate test occurs when we apply our knowledge to real-life situations. Whenever Git does not behave as expected, we find out where we need to improve the model.

Polish & refine

As mentioned in the introduction post, building mental models is a four-step process:

  1. Define your model. We all use some kind of model, whether we are aware of it or not.
  2. Test your model. Either by researching or in real-life situations.
  3. Improve your model. Take what you have learned in 2) and improve it. Then test again.
  4. Simplify or go deeper. There will come a moment when steps 2) and 3) won’t bring any further improvements or when the model does not go deep enough because you have advanced in your understanding and tackling bigger challenges. You may be at a point where you can significantly simplify the model or build a new one.

We have not yet exhausted steps 2) and 3), but we can still simplify our model. The table metaphor has gaps and is not that useful (but it helped to develop the model of snapshots).

Instead, the model of snapshots and the three file states together seem to be a good first mental model of Git.

The final Git mental model

Log of hashes & three states

  • Every committed change in Git is stored as a snapshot and referenced by a unique hash. A log of changes is represented by a log of hashes. To compare two branches, all we need to do is compare the hashes to know if they are the same or not.
  • All files go through three possible states.


While developing an understanding of how Git works, we have gone through the process of constructing mental models. The key takeaways are the following:

Always build your mental model first. Before you start reading a book or tutorial, sit down and visualise how you believe that the thing you are interested in works. That will make your subsequent research a lot more efficient, as you have a base that guides your reading and a place where you can place new information and relate it to existing knowledge. Remember the last time you read a book without any preparation. You may have highlighted sentences and written some notes in the margins - but how much of that information could you actually remember?

Test and refine your model. That step is easy to forget, especially when it comes to improving your mental model. When you hit a plateau in your learning effort, that is the moment where you need to go back to your model and work out where the gaps are.

Just enough information Good mental models are simple, some even elegant, and they convey just enough information to be useful.

The goal is not 100% correctness. A model is always an approximation and depends on your context and level of understanding. Don’t try to cram everything in it. Instead, figure out what are the most important facts of something. That exercise itself is very valuable to master a skill.


With the model at hand, we can now go and apply it to our daily work.

A good idea might be to look at your .git folder and try to figure out what is going on there. That will give you a deeper understanding of the internals (the relevant chapter in Pro Git is Git Internals Plumbing and Porcelain).

Or even better, don’t do anything and just use Git and wait until you hit a bump. That is the beauty of learning with mental models. There is no fixed path that you have to follow - real-life situations will show you where you need to improve.

I will come back to Git in a follow-up post and apply the model to more advanced situations, showcasing examples of where our model is sufficient and where it is not.