Albert and Aquinas agree in declaring that the principle of individuation is to be found in matter, not, however, in matter as a formless substrate but in determinate matter (materia signata), which is explained to mean matter quantitatively determined in certain respects.
But easiest of all is it to leave the relation of the unknowable "substance of Mind" to the unknowable "substance of Matter" (substance he throughout conceives as the unknowable substrate of phenomena) to the Unknowable, as he finally does.
Taking substance entirely in the sense of substrate, he argues that there is no evidence of a substantial substrate beneath mental operations; that there is nothing except unitary experience consisting of ideas, feelings, volitions, and their unity of will; and that soul in short is not substantia, but actus.
Secondly, when Wundt comes to the psychical, he naturally infers from his narrow Kantian definition of substance that there is no proof of a substrate over and above all mental operations, and falsely thinks that he has proved that there is no substance mentally operating in the Aristotelian sense.
Thirdly, on the grounds that logical thinking adds the notion of substance, as substrate, to experience of the physical, but not of the psychical, and that the most proper being of mind is will, he concludes that wills are not active substances, but substance-generating activities (" nicht thatige Substanzen sondern substanzerzeugende Thdtigkeiten," System, 429) What kind of metaphysics, then, follows from this compound of psychology and epistemology?
Kant, taking it in the mistaken meaning of Locke, converted it into the a priori category of the permanent substrate beneath the changes of phenomena, and even went so far as to separate it from the thing in itself, as substantia phenomenon from noumenon.
Fechner, Wundt and Paulsen have fixed the conclusion in psychology that soul is not substance but unity of mental life; and Wundt concludes from the modern history of the term that substance or " substrate " is only a secondary conception to that of causality, and that, while there is a physical causality distinct from that of substance, psychical causality requires no substance at all.