David Hume
Scottish philosopher and historian; he is one of the greatest philosophers in the English speaking world, at least, and to many people (including this writer), one of the greatest, without qualification.
His Treatise of Human Nature (first published anonymously in 1739) covered a wide range of philosophy, including epistemology, philosophy of science, philosophy of mind, and moral philosophy. His philosophy is basically reductionistic, in that everything gets its meaning from "ideas", and "ideas" in turn are derived from "impressions".
Especially important is his discussion of causality and induction, and many contemporary philosophers of science continually refer back to his view. What is causality, in the first place? Is causation reducible to constant conjunction of two events with temporal succession and spatial contiguity? Is necessity a subjective feeling? Is "inductive reasoning", which seems indispensable for obtaining causal knowledge, justifiable on rational grounds? These questions recur again and again in a new form in the contemporary philosophy of science and epistemology.
Hume's discussion of moral philosophy, mainly in terms of justice and property, is also important. Kant's metaphysics of morals seems to have been written in view of Hume's position, but this writer thinks Hume's view is better than Kant's in many respects.