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> But I'd guess that at least 10 of those 100 developers might say something like "business logic belongs in framework-agnostic pure functions, unless you have a good reason not to."

Those 10 developers might have a decent answer to the question "Where should business logic live?" (decent depending on other design considerations).

But that does not answer the question I asked them, which was "What is business logic?"

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You're right. And I realize I can't answer the question. But I wonder how much it matters? If a colleague and I disagree on whether some authorization logic counts as business logic or not, what is at stake?

Business logic is often the type code that changes least. If it is well designed, isolated and organized your business logic can survive UI changes, framework changes, stack changes, database vendor changes and all sorts of other "infrastructure" change. If you can't identify that "layer" from the others, then business logic will make its way into all sorts of areas that make change harder. And the ability to change is the very value that software has to offer in the first place. If it didn't need to be resilient to change then we could stick with fixed circuits which are far less expensive to build and maintain.

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