The Great Mental Models Vol. 2 by Shane Parrish
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Short Summary

The second book of The Great Mental Models series brings to life the core ideas from physics, chemistry, and biology. If you want to make better decisions, it starts with understanding the ideas that govern how the world works.

Favorite Quote

"Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less."

Marie Curie

Book Notes

Volume two of The Great Mental Models series explores the core mental models derived from the fundamentals of science: physics, chemistry, and biology. Understanding these models will help you improve your understanding of how the world works so you can learn to make better decisions.

Part 1: Physics

"Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less."

—Marie Curie


Relativity helps us to understand that there is more than one way to see everything. An effective tool for helping us see things from different perspectives is the thought experiment.

Thought experiments help us understand why people do things that initially don't make any sense. When you understand someone's point-of-view, it will help you better understand their beliefs and biases that guide their worldview.

You can also solve problems by shifting your perspective in multiples ways:

  • Zooming in or out of the problem
  • Extending your timeline to weeks, months, or even years
  • Assuming the perspective of other stakeholders

Nobody can see it all. Filtering a problem through multiple perspectives layered together will help reduce your blind spots and offer you a version of reality that’s closer to the truth.


We are more strongly driven to avoid losses than to achieve gains. It’s why we find it so difficult to put ourselves out there to engage with others. But life is an iterative and compounding game. In the words of Peter Kaufman, "It pays to go positive and go first."

Reciprocal behavior is embedded in our DNA. You were more likely to survive if you received help from others, and you were more likely to receive help if you offered assistance.

There are two types of reciprocity that we engage in: direct and indirect.

  • Direct: You help me, and I help you.
  • Indirect: The pay-it-forward concept. I help you, and you help someone else.

Life is easier and more enjoyable when we act on starting and maintaining win-win relationships. So, if you want to receive help, start by first offering help to others.


Thermodynamics refers to a set of four laws that provide the ultimate foundation for how the world works:

  1. The First Law: energy cannot be created nor destroyed; it can only be transferred from one form to another.
  2. The Second Law: Entropy (the measure of disorder) of an isolated system always increases.
  3. The Third Law: As temperature approaches absolute zero, the system’s entropy approaches a constant value.
  4. The Zeroth Law: If two objects are in thermal equilibrium with a third object, then those two objects are in thermal equilibrium with each other.

Thermodynamics teaches us that chaos is the default state. It requires energy to maintain structure and organization. We put in the work to maintain order because we don't like the idea of living in a world that we cannot fully control or understand.

We tell stories to ourselves and others as a way of filling in the gaps of our understanding. Stories attempt to tame the terrifying randomness surrounding us so that we can maintain social order and cultural norms. Without our collective belief in stories (money, government, borders, etc.), our complex societies would collapse.


Inertia is useful for understanding some elements of our behavior, including our thinking patterns and habits. We're naturally inclined to reject the new in an attempt to resist the effort required to make a change. Keeping things as they are requires almost no effort and involves little uncertainty.

Real change requires force, and force requires effort. The longer you've been doing something, the more it becomes a part of your identity. Thus, the amount of effort needed to change a habit is proportional to the length of time you've had it. The longer a habit has been around, the more energy is required to change it.

We naturally want to conserve our energy. That’s why getting started is the hardest part. But once you get moving in a direction, it's much easier to keep going.

Friction and Viscosity

Friction is the force that must be overcome to achieve an outcome. It’s what opposes the movement of objects that are in contact with each other. Viscosity is the measure of how hard it is for one layer of fluid to slide over another.

While often hidden, friction and viscosity work against us anytime we try to do something. We often default to using more force to overcome resistance when merely reducing the friction or viscosity will do. Sometimes all it takes to achieve your goals is to think about how you can reduce the resistance instead of just trying to apply more force.


Speed is just movement; velocity has direction. It's much more important to pay attention to where you're going instead of how fast you're going. Progress in a given area is not about how fast you're moving now but how far you've moved relative to where you started.

Self-improvement is about finding a balance between speed and direction. Learn to improve your tactics and be willing to adjust and respond to new information. Sometimes you won't move as fast as you want, but it's better to move in the right direction at a steady pace than to go fast in the wrong one.


Leverage is achieving results far greater than the force you put in. When it comes to leverage, you want to know three things:

  1. How do I know when I have it?
  2. Where and when should I apply it?
  3. How do I keep it?

If you can figure out those three questions, you can have significant power over the forces acting against you. But as powerful as leverage can be, you need to be deliberate about using it. Abusing your leverage can lead to others feeling exploited and not want to work with you.

Leverage is best paired with reciprocity—building win-win relationships that will help you keep your leverage sustainable over the long-term.

Part 2: Chemistry

"Science and everyday life cannot and should not be separated."

—Rosalind Franklin

Activation Energy

Activation energy is the energy required in a chemical system to initiate a reaction. Without an initial input of sufficient energy, the reaction won't occur.

When it comes to achieving lasting change in your life, you need enough activation energy to break out of old habits. Real change requires more than just enough energy to get started, but enough energy to see the entire process through. The bigger and more challenging the need for change, the more activation energy is required.

Figuring out the right amount of energy is pertinent to quitting some addictions. It's not just the moment you decide to quit; it’s everything that had to happen, and every crisis you had to face to enact that decision.

Real change takes effort. It requires not just updating your goals but also changing your systems. Invest more energy than you think you need to, and you just might get there.


Catalysts accelerate change. They are a part of a reaction, but the reaction does not consume them. Their job is to create an alternate pathway for a reaction to occur that’s usually faster and easier.

Catalysts can just as easily speed up a negative reaction as they can a positive one. They just decrease the amount of energy required to cause change, and in the process, make certain reactions possible that might not have happened otherwise. For many people, unpleasant events, such as losing a job or being rejected, act as necessary catalysts for tremendous personal growth.


Alloying is the process of combining components in specific combinations to produce a substance that is greater than the sum of its parts.

Our knowledge can be viewed as an alloy since it’s a combination of knowledge gained from direct experience and knowledge gained from theory. Theory can trigger new experiences, while experience can trigger the updating of our theory.

Our knowledge can be further broken down into what Aristotle discussed as our five components of knowledge:

  • Episteme: scientific knowledge
  • Techne: art or craft knowledge
  • Phronesis: prudent or practical knowledge
  • Nous: intellect or intuitive apprehension
  • Sophia: wisdom

Alloying is about increasing our strength by combining multiple skills and specific knowledge. Combining expertise in a domain with a broader understanding of the rules that govern the world is a rare combination that will make you highly valuable in society.


"A totally blind process can by definition lead to anything; it can even lead to vision itself."

—Jacques Monod

Evolution Pt. 1: Natural Selection and Extinction

Evolution explains the relationship between us and our environment. Natural selection and extinction are powerful models because they demonstrate that we must either respond to our environment’s changing demands or risk dying off.

Natural selection is about what advantages you have in the current environment, not what advantages you might have in the distant future. It doesn't preserve changes that might be useful in the future; it preserves changes that are useful right now. If the environment changes, then you must learn to adapt.

If you want to understand why some traits stick around, why some customs carry through multiple generations, and why some ideas take root and spread, you have to look at their usefulness in their environment.

Evolution Pt. 2: Adaptation Rate and the Red Queen Effect

Adaptation refers to both the useful trait and the process of change it undergoes as it's passed on. Adaptations are genetic mutations that happen to occur at the right place and time. When they provide an advantage, the frequency of that mutation in the population increases.

The Red Queen Effect is an evolutionary principle that explains the pressures that all organisms face just to survive. When enough people are trying to get smarter, better, and more of the limited resources available, it puts direct pressure on everyone else to keep up.

It doesn’t matter how long a species has already survived; it must be willing to adapt or risk extinction. Real success comes from being flexible enough to change, even if it means abandoning what worked in the past, so that you can focus on what you need to do to thrive in the future. Complacency is what kills you.


Nothing exists in isolation. Everything is connected. Every decision you make may have unintended consequences. Therefore, take the time to learn how your system’s components are interconnected so you can understand how your actions will impact those connections and affect the outcome you're trying to produce.


Every species in an ecosystem has a niche. A species' niche includes everything that affects its ability to reproduce and survive. Generalist organisms have a broad niche, which means they can survive in a variety of places. On the other hand, specialist organisms need stable environmental conditions to thrive but tend to have few competitors.

Generalists face more daily competition but are more adaptable. Specialists have less competition and day-to-day stress, but only during times of stability. As soon as their environment starts to change, their stress skyrockets as they struggle to adapt.

The fittest species are the ones most suited for their environment and more adapted than their competition. Species will typically become more specialized as competition increases or risk becoming dislocated or even extinct.


We have an innate desire to preserve ourselves, whether it's by passing down our genes by having children or by trying to leave behind a legacy. Self-preservation helps us understand why we sometimes engage in counterintuitive actions such as sacrificing short-term guarantees for long-term possibilities.


Replication, whether through mitosis or meiosis, allows for diversity in traits that can improve our fitness and increase our chances of survival. Replication prevents the accumulation of traits that impair fitness and lets us, as a species, try out new behaviors that could give us an advantage.

Sometimes you don't need to reinvent the wheel. It's often a good starting point to copy what others are doing. Once you get a better sense of the environment, you can adapt to suit your needs and interests. Effective replication requires enough structure and space to produce a copy but enough flexibility to adapt that copy to changes in the environment.


What separates humankind from other species is our willingness to work together and collaborate on projects and ideas that otherwise wouldn't be possible if we went at it alone. Cooperation teaches us to work together by asking for help when we need it and offering it in return.

Our complex societies are built on our ability to work together, believe in the same ideas, and share the same goals. When we cooperate, we lighten our load and create new things that move society forward.

Hierarchical Organization

Hierarchies are found across the animal world, and humans are no different. The way we organize ourselves is often a default to our instincts on leadership and authority. Our hierarchical organizations are where we derive our ego, status, and reputation, and they condition us to focus on growing ourselves rather than growing others.

Even in the absence of an imposed structure, we instinctively self-organize. That explains why even anarchist movements end up with leaders. Since hierarchies are a core human instinct, we must learn to be aware of them and learn how to work with them, not go against them.

Hierarchies are also critical in survival situations and combat. In times of extreme stress and chaos, we naturally crave leadership. We are all looking for leaders, even if it means we are looking at ourselves.


Incentives are a powerful force that can’t be underestimated. We instinctually move in the direction of rewards and do our best to take steps to avoid punishment. When we’re thoughtful about incentives, we can modify our behavior or the behavior of others.

Our behavior is continuously changing based on both the actual reward and punishment and our perceptions of it. Becoming aware of the incentives that may be directing our actions can help us recognize any unfavorable influences so that we can refocus our attention on what we value.

It always pays to consider the incentives influencing our choices. We often tell ourselves that our motivation is based on doing the right thing when actually we are incentivized by the allure of rewards. Knowing how incentives work to motivate us can help us be less easily manipulated.

Tendency to Minimize Energy Output

All living things require energy to perform their daily functions. Over time, species have developed different mechanisms to increase their energy efficiency. We’ve evolved to conserve our energy so that we have enough to draw from when we really need it, such as when being chased by a predator.

The problem is that our modern society no longer reflects the environment in which our evolutionary tendencies were developed. Our instinct to minimize energy output can lead us to be resistant to change or avoid taking non-life-threatening risks.

Understanding that we're naturally lazy creatures can help us better understand our tendencies and why it's difficult for us to change our minds. If we want to improve our thinking and get the most out of our environments, we have to be aware of our natural tendency to minimize energy output and correct for it where doing so creates value.

Read more on Amazon:

The Great Mental Models Volume 2: Physics, Chemistry and Biology