A razor floating in the cosmos.
By various unknown artists via Stable Diffusion and Stable Diffusion Infinity

Occam’s Deepest Cut

The razor doesn’t just point at truth: it defines it

Naim Kabir
6 min readJan 6, 2023

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The planets orbit the Sun.

This is something we’d call a “true fact” about our universe. It overturned a previous false fact: that everything in our sky revolved around the Earth in nested cycles and epicycles.

But why did we overturn the older Ptolemaic model with Copernicus’ heliocentric solar system when it only produced equivocal gains in predictive power?

In his Structure of Scientific Revolutions, Thomas Kuhn writes:

Ptolemaic astronomy is still widely used today as an engineering approximation; for the planets, Ptolemy’s predictions were as good as Copernicus’.

If accuracy of the theory wasn’t what clinched our collective decision, what was?

A side-by-side comparison of the two models gives us a clue.

Left: James Ferguson’s (1710–1776) rendering of the Ptolemaic model for Encyclopaedia Britannica; Right: Nicolaus Copernicus’ diagram of a heliocentric model for De revolutionibus orbium coelestium

The Ptolemaic model is just more complicated. It features dizzying spirograph paths that are muddled and hopelessly confused.

Meanwhile, the Copernican model gives us a few clean orbits.

This elegance is what made us overturn what we once called “true”.

Kuhn again:

Given a particular discrepancy, astronomers were invariably able to eliminate it by making some particular adjustment in Ptolemy’s system of compounded circles. But as time went on, a man looking at the net result of the normal research effort of many astronomers could observe that astronomy’s complexity was increasing far more rapidly than its accuracy

Astronomers could make the Ptolemaic model work, like lazy engineers slapping spaghetti code on a legacy system—but it was complex. Because it was complex, it was brittle: complexity invited…

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Naim Kabir

Engineer. Focused on experimentation, causal inference, and good software design. naimkabir.com