Hillary Putnam is a key figure in the
philosophy of mind during the
20th century. He was an early and influential advocate of
functionalism (roughly, the view that the human mind is analogous to a digital computer) but in "Representation and Reality" he recanted his earlier views and set out to explain why functionalism, in his revised view, would not work. He is well known for his doctrine that "meaning is not in the head", which is most famously illustrated by his
Twin Earth thought experiment.
Putnam also contributed to the resolution of Hilbert's tenth problem in mathematics.