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Simple model vs complex model — which one to use?

You might have heard Elon Musk talk about first principles or read Principles by Ray Dalio.

Principles help us comprehend the behaviors of a system. It this happens, then that happens. They help us understand increasingly bigger chunks of a system and then translate that understanding into action — creating expertise.

But arguably, neither Elon Musk nor Ray Dalio have a principial understanding of dieting, or plant growing, or carpentry. They might know some basics and have a couple of simple rules concerning those things, but they don’t have a complex model for those areas and they don’t need to have it in order to be successful.

Build and use a complex, detailed, principial model of a domain if your livelihood depends on it or you’re absolutely fascinated by it.

Use a simple, basic, heuristics-driven model for everything else. You don’t need to understand the principles behind everything. Trying to do that will cost you effort better spent on building the robust models you need.