Empiricism holds that knowledge of the world comes from experience. Whatever content a concept has can be traced back to something seen, heard, touched or felt, and claims that cannot be traced that way are suspect.

The British line

John Locke opened An Essay Concerning Human Understanding (1689) by attacking innate ideas. The mind, he argued, begins as a blank sheet — tabula rasa — and is written on by sensation and by reflection on its own operations.

George Berkeley pressed the principle further. If everything known of an object is the ideas it produces, there is no ground for positing matter behind those ideas, and to be is to be perceived.

David Hume drew the skeptical conclusions. Tracing every idea to an impression, he found no impression corresponding to necessary connection between cause and effect, which produced the problem of induction, and no impression of a continuing self, which produced bundle theory.

The argument with rationalism

Rationalism holds that reason reaches substantive truths without experience, and points to mathematics. Empiricists reply that such truths are about the relations of ideas and tell you nothing about the world until experience supplies the subject matter.

Kant accepted Hume's challenge and answered that the mind contributes the form of experience — space, time and the categories — while experience supplies the content. Knowledge that is both informative and known in advance becomes possible on those terms, which is his synthetic a priori.

What it became

Empiricism shaped the modern scientific outlook and, in the twentieth century, logical positivism, which held that a statement is meaningful only if it can be verified. That criterion proved too strict to survive, including on its own terms, but the underlying demand — say what would count as evidence — remains one of the most useful habits the tradition produced.