While in the chapter on empirical psychology Wolff merely observed the mental capacities of human beings, in the chapter on rational psychology he tries to determine the nature of the soul or the thing that has such capacities. Nowadays it is easy to think such an attempt as completely ridiculous: after all, Kant should have shown that rational psychology was based on mere sophistic reasoning. We shall have to speak of the Kantian criticism in the future. At this point we may just note the surprising fact that Wolffian theory of the soul begins with a reasoning of transcendental nature. That is, Wolff begins from the fact that human beings are conscious of themselves and investigates the presuppositions of this fact.
The beginning of Wolff's reasoning is innocent enough. We are conscious of something, Wolff says, when we can distinguish it from other things. For instance, I am conscious of a hand mirror, only if I am able to differentiate the mirror from e.g. hands holding it. Particularly, consciousness of oneself implies the capacity to distinguish between oneself and other things. In other words, consciousness is connected with a clarity of thoughts, which for Wolff meant a capacity to distinguish the object of thought from other objects.
Furthermore, in order to distinguish different objects from one another and to recognise them as different, one must be able to contrast them with one another and to consider them one after another, Wolff continues. In order to do this, the conscious being must have imagination and memory, that is, he must be able to think of things that are not present and to recognise them as having been present. Indeed, consciousness is generated, because we can think of a thought for a period of time, note that the time has changed, but the thought itself has remained.
The arguments thus far have been essentially about characterising what it means to be a conscious personality: e.g. a person needs to have a memory, in order that she would have a sense of continuity of self. I cannot see why Kant or his followers would have any reason to argue with these considerations. In fact, much of the Kantian and post-Kantian German philosophy consists of such theorising on the nature of human consciousness, although in a somewhat deeper level.
What Kant and his followers would probably find unacceptable is the next move where Wolff tries to prove the immateriality of soul or the thing that is conscious. Wolff starts by noting that all material processes must be explicable through mere mechanical movement of material objects. Thus, if soul would be material, thinking would also be such a mechanical process. Now, human beings can be conscious of their own thinking, that is, they can note that the starting and the end point of thinking are different and still parts of a continuous process of one thing. Here Wolff simply states that such representation of continuity is impossible with mere matter: material objects as complexes can at best represent only other complexes, but they cannot represent a unified process of thought.
Wolff's blunt statement that matter as a mechanism cannot represent thought processes is quite unsatisfactory, especially as we nowadays don't think that matter consists merely of lego-like blocks that interact only through mechanical contact. Wolff does try to amend his reasoning by noting that material object can represent things – for instance, we can make a clay model of a building – but it cannot represent the original as separate from the representation. I have a feeling that Wolff has here confused first- and third-person perspectives. Surely an external observer cannot see e.g. brain as a representation of the process of thought,but this does not mean that the brain could not represent this to itself, if it just were conscious.
The situation appears even worse, when we consider that by denying the complexity of the soul Wolff has to accept the possibility of a simple thing representing complexities, which appears at least as difficult as a complex representing a unity. Indeed, how could a single partless entity represent a complex of many entities correctly? The only possibility appears to be that the complex is characterised through passage of time: at one point soul represents one part of the complex, at another point other parts and at final point the combination of the two previous phases. Indeed, we might well believe that human consciousness does work in this manner: e.g. when looking at a boat moving at the sea, we concentrate first on its rear end and then on the other end and only after that note that both parts go together.
The problem with this solution is that it once again threatens the supposed unity of consciousness. Suppose that I am thinking of myself. This act of thinking then represents some state of my mind, say, a memory of yesterday. But this act as simple can only represent one thing, thus, it cannot represent itself. We could then begin a new act of thinking that had the original act as its object. This procedure could be iterated indefinitely and so a problem is revealed: no matter how far we'd go, there would always be left at least one act of thought that had not been combined with other acts, that is, the very act of thinking all the other thoughts. This problematic was something that intrigued some of the later German philosophers, although Wolff appears to not have noticed it.
Wolff's argument for the immateriality of the soul was then unconvincing. Next time we shall see whether he can at least characterise this supposed immaterial entity.