The “old logic,” which was abandoned for the most part after John Stuart Mill (1806-1873), was grounded in introspective psychology: the study of subjective experience. By contrast, the “new logic,” Gottlob Frege (1848-1925) being one of the best and most representative examples, is not grounded in introspective psychology. For example, David Hume (1711-1776), who Mill took seriously, asked the introspective-psychology question (paraphrased): “What’s the subjective experience of belief? It’s possible for me to imagine that my paternal grandfather is still alive—to visualize him, say, waking up in the morning and then cooking breakfast—without believing that what I’m imagining is true. It’s also possible for me to imagine the same thing but as if it’s true. What’s the difference between the former and latter subjective experiences, between imagining-believing and imagining-non-believing?” Who still dared to ask that question after Mill?

Isaac Newton (1643-1727) tried to figure out what the mechanism is of gravity, failed, and then published something less ambitious: only a description of the “what” of gravity (as opposed to also an explanation of the “why”). Newton himself was unsatisfied, but the Newtonian revolution got the scientific world used to description without explanation.

Interestingly, Noam Chomsky (1928-) intentionally did for linguistics what Newton unintentionally did for physics. He didn’t even try to figure out what the mechanism is of grammar (which made introspective psychology irrelevant).