 Setting aside what it is properly called, the thesis of physicalism is often described as an extremely old, even ancient, thesis. The first sentence of  Friedrich Lange’s  The History of Materialism, which was the standard work on the subject in the 19th century is: “Materialism is as old as philosophy, but not older” (1925, 3). What Lange has in mind is the pre-Socratic philosopher  Democritus,  who is usually thought of defending a kind of physicalism or materialism when he said or allegedly said, “all is atoms and the void.” This view casts a long shadow over subsequent formulations of physicalism. A position like that of Democritus was revived in the early modern period just prior to Newton, by philosophers and scientists such as  Hobbes  or  Gassendi.  In the eighteenth century, French philosophers like  D’Holbach  and La Mettrie thought of themselves as materialists (and would be now classified as physicalists) somewhat in the same way since they held that each human being is a complicated sort of machine. In the nineteenth century, while  Karl Marx’s  ‘dialectical materialism’ is something different from the metaphysical thesis we will focus on, he nevertheless developed his social philosophy against the background of what we would call physicalism; in fact, Marx’s doctoral dissertation was a comparison of Democritus and  Epicurus.  And in the twentieth century,  analytic philosophers such as Smart and Lewis self-consciously  defended their views in a way that acknowledges, as Lewis put it,  “our intellectual ancestors” (1994, 293).