This philosophy, called Empirio-criticism, is not, however, a realistic but an idealistic dualism, nor can it be called materialism.
Dualism meets the assertion of absolute unity by the counter assertion of mere difference.
So far from establishing the truth for which dualism is itself concerned - the reality of all differences - such a theory can end only in a scepticism as to the reality of any difference.
The psychological theory of cognition takes for granted the dualism of the mind that knows and the object known; it takes no account of the metaphysical problem as to the possibility of a relation between the ego and the non-ego, but assumes that such a relation does exist.
It might seem, indeed, that Stoicism indicates a falling off from Plato and Aristotle towards materialism, but the ethical dualism, which was the ruling tendency of the Stoa, could not long endure its materialistic physics, and took refuge in the metaphysical dualism of the Platonists.