Qualy #65 - The three laws of medicine — Law #1: A strong intuition is much more powerful than a weak test

**Peter Attia** (0:05)
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I want to talk about another book you wrote that doesn't get as much attention, which is The Laws of Medicine. You wrote that after Emperor Before the Gene, correct?

**Siddhartha Mukherjee** (1:11)
That's right. So Laws of Medicine has a very different mandate, as it were, and that's because the book came out in association with TED. They had commissioned 10 books by 10 thinkers around the world, and they asked me to write a book on that, and it's necessarily a small book.
It's really, the mandate was to write basically a 75-page book, expanding on a single, very, very incisive idea. So that's the Laws of Medicine, yes.

**Peter Attia** (1:36)
If I got them correctly, the three laws are, a strong intuition is much more powerful than a weak test. How did you think of that, and what is the most important application of that law to the way you think about medicine, or specifically oncology today?

**Siddhartha Mukherjee** (1:49)
This, to me, is one of the great neglected ideas in medicine, perhaps one of the great neglected ideas in the world. This idea initially comes from Thomas Bayes. This is a Bayesian idea. Thomas Bayes was a cleric, but by evening, he was a mathematician and an economist, and his work leads to one of the most seminal and funny thought experiments that I've ever encountered, which is the following, and I sometimes quiz my daughters with it, which is the following. This is not Thomas Bayes' own example, but it arises out of Thomas Bayes' work, and one might imagine going to a street fair and encountering a man who's tossing coins. And he tosses coins, and your job is to predict whether the next flip, coin flip, is gonna be heads or tails. And so he tosses the coin 20 times, and all 20 times it's tails.
So then he turns to the crowd, and he says, what's the next coin flip going to be heads or tails?
Now, the mathematician in the crowd who's the professor of mathematics.
It says 50%, and everyone says absolutely right.
But the child in the crowd says, no, no, you don't understand. This is a stupid problem. It's the coin's rigged.
The coin has only, it has two heads or two tails as the case may be. And the child's right. And what's important about that insight is that the mathematician imagines the world, this in this case, this is not a stab at mathematicians in general, but the professor of mathematics thinks of the world as having no history, as having no a prioris. It's a world that's created de novo every time. The coin is flipped and its heads and tails equal every time. But the child knows, and all humans know, that in fact the world doesn't behave like that. Everything has priors. And you need to understand those priors before you can understand the posterior. There's wisdom in that idea.
And it took someone like Thomas Bayes to figure that out, that most of our lives, we aren't living our lives like the crazy mathematician professor. We are living our lives like the child. We're thinking to ourselves, well, what was the prior antecedent? Imagine this is true for any corner of your life. The first question you ask yourself when you're trying to solve a problem, trying to understand the cosmos, trying to understand something, you ask yourself, well, what was the prior like? Did the sun set in the west last night? And how about the night before? And maybe I don't need to create a formula to figure out whether the sun is gonna set on the west or the east tomorrow. It's because it's set on the west every time. There are obviously loopholes and gaps to this kind of thinking. There are surprises that you can miss. So Bayes' fundamental idea was that you can only interpret a test in the light of what that test is predicted in the past. It's an extraordinarily important idea in the way we think about the universe. That the past performance of a test tells you something, not everything, but tells you something about the future performance of a test. And you can apply it to many, many things in the world. You can apply it to any kind of thinking, economic, economic thinking, climate change-oriented thinking, that the past is a guide to the future, not only in a kind of loose way, but you're really using a rheostat weighted strongly by the past. And this, of course, applies to medicine. And it's a forgotten rule in medicine.