The American logician, mathematician, and philosopher Charles Sanders Peirce (1839–1914) was a scattered but extraordinarily deep thinker who never gained the recognition he deserved in his lifetime. In his 1878 paper "How to Make Our Ideas Clear," he gave philosophy a lasting method: the pragmatic principle.

Peirce's question was: what does it really mean to understand the meaning of a concept? His answer was radically practical. Our thought about an object or idea is, in fact, nothing but our thought about the observable consequences it may produce. To say a thing is "hard," for example, is to say it will not be scratched when you try to scratch it. If you list all the practical effects of a concept, you have set out the whole of its meaning; there is no hidden "essence" beyond these effects. Most metaphysical disputes, Peirce said, are empty precisely because they make no observable difference.

This principle redefined the concept of "truth" as well. For Peirce the truth of a belief lies not in some abstract correspondence but in whether inquiry, in the long run, will confirm it; truth is what an ideal community of inquiry would ultimately agree upon. Knowledge is not the intuition of a single genius but a shared, correctable process that advances by weeding out errors.

It was his friend William James who brought Peirce's ideas to a wide audience; but James drew pragmatism more toward individual benefit and psychology, and Peirce, objecting, used the name "pragmaticism" to set apart his own view. In later generations Dewey carried it into education and democracy. With an influence reaching from the philosophy of science to the theory of meaning and to semiotics (Peirce is also a founder of modern sign theory), Peirce stands at the foundation of America's most original contribution to philosophy.