Let us mark clearly how this union is put forward by us. We describe it after nature. It is observation which reveals to us the union and the fusion of the two terms into one. Or, rather, we do not even perceive their union until the moment when, by a process of analysis, we succeed in convincing ourselves that that which we at first considered single is really double, or, if you like, can be made into two by the reason, without being so in reality. Thus it happens that we bring this big problem in metaphysics on to the field of observation.
Our solution vaguely resembles that which has sometimes been presented under the ancient name of _physical influx_, or under the more modern name of _inter-actionism_. There are many authors who maintain that the soul can act directly on the body and modify it, and this is what is called inter-actionism. Thereby is understood, if I mistake not, an action from cause to effect, produced between two terms which enjoy a certain independence with regard to each other. This interpretation is indubitably close to ours, though not to be confused with it. My personal interpretation sets aside the idea of all independence of the mind, since it attributes to the mind an incomplete and, as it were, a virtual existence.
If we had to seek paternity for ideas I would much rather turn to Aristotle. It was not without some surprise that I was able to convince myself that the above theory of the relations between the soul and the body is to be found almost in its entirety in the great philosopher. It is true that it is mixed up with many accessory ideas which are out of date and which we now reject; but the essential of the theory is there very clearly formulated, and that is the important point. A few details on this subject will not be out of place. I give them, not from the original source, which I am not erudite enough to consult direct, but from the learned treatise which Bain has published on the psychology of Aristotle, as an appendix to his work on the Senses and the Intelligence.
The whole metaphysics of Aristotle is dominated by the distinction between form and matter. This distinction is borrowed from the most familiar fact in the sensible world--the form of solid objects. We may name a substance without troubling ourselves as to the form it possesses, and we may name the form without regard to the substance that it clothes. But this distinction is a purely abstract one, for there can be no real separation of form from matter, no form without matter, and no matter without form. The two terms are correlative; each one implies the other, and neither can be realised or actualised without the other. Every individual substance can be considered from a triple point of view: 1st, form; 2nd, matter; and 3rd, the compound or aggregate of form and matter, the inseparable _Ens_, which transports us out of the domain of logic and abstraction into that of reality.