Aspects of the Theory of Syntax did not present discoveries about language that have since been confirmed as correct by subsequent scientific work. ... Yet it was truly a wonder. Not for what it claimed in detail, but for what it led to.
The celebrated page 3 of Aspects of the Theory of Syntax (1965) announces that the actual subject matter of linguistics is the intuitions about sentence structure of an imaginary “ideal speaker-listener in a completely homogeneous speech-community, who knows its language perfectly and is unaffected by such grammatically irrelevant conditions as memory limitations, distractions, shifts of attention and interest, and errors (random or characteristic) in applying his knowledge of this language in actual performance.”Pullam went on to quote a recent Economist article which pointed out that Edward Lorenz, the pioneer of applied nonlinear dynamics, once distinguished weather from climate not in terms of averaging out daily weather over some longer period, but in a conceptually simpler way: “Climate is what you expect; weather is what you get.”
This led GP to write: Competence is what you expect; performance is what you get.
Again: (Linguistic) competence is what you expect; performance is what you get.