This is the heart of the ‘anti-private-language’ argument in his Philosophical Investigations (published posthumously in 1953), one of the most celebrated arguments of twentieth-century philosophy. Wittgenstein tried to show that there could be no significant thought about the nature of one’s past (or future) mental life if that mental life is divorced from the physical world in the way that Cartesian dualism proposes. It becomes, as it were, too slippery or ghostly even to be an object of our own memories or intentions.
