First Principles Interviewing

In software engineering, there’s a famous interview question that simply asks: You open your web browser, type in “” and hit Enter. What happens next?

A student might talk about how your browser checks a cache and then asks a DNS server for the appropriate IP address to send a request to. A veteran engineer might go into far more detail about recursive vs root nameservers, Google’s backend load balancing or CDNs, TLS authentication, details of JS libraries, etc.

Of course, if you really wanted to, you could even describe how light in fiber optic cables is multiplexed to physically deliver data to your local network. The purpose of the question is to see how far down the rabbit hole you can go.

As it turns out, this is an excellent category of interview question for all roles. I recommend that founders build an intentional “First Principles Question” into their interviewing process to weed out the “intellectual yet idiot” types that are smart on paper but not in practice.

This is not something you need for all roles, but is important for any role requiring genuine innovation, complex problem spaces, interdisciplinary work, or critical decision making. Anybody can study for an interview and come prepared or polished, but nobody can fake understanding and sound first principles.

Examples and Best Practices

My friend Saneel interviewed for a mechanical engineering role at a great “hard tech” startup. During the final round talk with the CEO of the company, he was asked two questions.

First, he was asked to derive the equation for a line from scratch. “y = mx + b” is such a simple equation that we were taught back in middle school. But where does this actually come from? If you’re curious, I’ve linked one solution here.

Apparently many PhD engineers who have done advanced math still struggle with this question. Asking yourself “what really is a line” is not something most people have done before.

The second question was to name an exponential equation. Everyone technical knows “e^x” from the natural logarithm or Euler’s identity. My friend gave that as his answer. The CEO then said “ok name another one.”

Similarly, it can be surprisingly hard to think of a second exponential equation that gets used in the real world. If you’re the sort of person who draws knowledge from your homework and exams, nothing may come to mind. If you’re the sort of person with an expansive yet intuitive grasp of mechanical engineering, you might be able to think of another exponential pattern somewhere.

Each business has different needs. That said, here are a handful of questions I might ask someone depending on the circumstance. Feel free to use these as inspiration for your own purposes.

Each question is very straightforward, but surprisingly difficult to answer completely and correctly:

  • Why has natural selection never created an immortal organism?
  • Why does the stock market value grow much more quickly than GDP? 7% vs 2% is a vast gulf in the world of compounding. In fact, since it’s possible to produce more goods without actually making a profit, you would think that GDP grows faster.
  • What does a parabola, aka quadratic equation (ax^2 + bx + c), describe?
  • Why does kinetic energy increase quadratically with velocity instead of linearly? You would intuitively expect it to be linear like momentum.
  • Why does anyone bother to vote given that no individual will influence an election?
  • Why are capital gains taxed differently from earned income? There are actually several reasons for this. One of them being that capital investments are diluted by inflation over time, which should be accounted for fairly.
  • How would you design a governance system for a Mars colony?

What I plan to do as we hire other investors at Contrary is have an entire interview where we talk about something wholly unrelated to startups and investing. If someone studied philosophy in college, go deep on a topic there. If they worked in DC, get their take on the political landscape. Some companies already do this, but very few keep digging until they hit bedrock.

My last comment here is that taking First Principles Interviewing seriously can be an important cornerstone of your company culture. Interviewees are constantly evaluating you as a company just as you evaluate them as a candidate. Great people want to work with great people, and taking the effort to talk through foundational questions can breed a culture of learning, ambition, and clarity.

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