Ever since Descartes suggested that the finite world consisted of two types of substances, material and spiritual, the question of possible interaction between the two had formed a dilemma. Descartes' own suggestion that pineal gland had something to do it was considered a failure. An easy solution would have been to get rid of one side of the equation altogether. But the rejection of spiritual substances or souls, that is, materialism of Hobbesian sort was considered antireligious. On the other hand, the opposite of materialism, which denies the existence of matter and which Wolff called idealism, attracted religious men like Berkeley, but was otherwise seen as rather farfetched.
Spinoza's solution was to deny that there is any true difference between soul and matter: both are merely one and the same thing from two aspects, thus, they do not interact, although changes in one reflect the changes in the other. But Spinoza's answer led to a pantheistic world view, where everything was a mere modification of original unity or God. A more theologically acceptable solution was occasionalism, according to which God had to give a helping hand, whenever an apparent interaction of any substances would occur. Problem was that occasionalism required constant wonders and so undermined scientific discussions.
Wolff follows Leibnizian solution of the problem, that is, he supposes that God had created the material world and the souls in such a manner that they appeared to work in harmony with one another. For Leibniz and Wolff, soul was closed off from any influences and all its states followed from its previous states. Still, soul could represent the world around it, because when the soul had been generated by the God, it represented the world perfectly and thereafer, because of the laws governing both matter and souls, the two will remain in perfect sync (one might object that two clocks that begin by showing the same time might not be synchronised after few days, but I will assume that God has ordained the laws in question so that the harmony remains).
Leibniz's theory is undoubtedly ingenious, but it somehow feels too elaborate. Furthermore, it still appears to verge on materialism. The body is not truly controlled by the soul, thus, whatever words are coming out of the mouth of the person sitting next to me, whatever actions she will perform – all this must be caused by some changes in her body in general and her brain in particular. I know that in myself there is something more – namely, my consciousness that exactly corresponds to the actions of my body – but in case of other persons I might as well assume that they are mere machines.
Leibniz and Wolff had, of course, a reason for adding independent souls to the equation. The material bodies can be destroyed through disassembly of their parts, but partless and simple soul cannot be disassembled. Thus, human consciousness should live on after the death of the body.
What is most unsatisfactory in this account of the immortality of the soul is that it apparently fails in its purpose. True, Wolff and Leibniz do conclude that the soul is immortal. But the connection between the soul and its body has been defined to be very tight: what soul perceives, what it imagines and what it thinks all correspond to some states in the body of the soul. Indeed, Wolff even goes so far as to admit that a fault in person's brain will lead to a fault in the corresponding perception of the soul. It would then seem reasonable that the capacities of the soul would be gravely diminished when its body completely ceased to exist.
Here Wolff relies on some outlandish speculations. He assumes it to be proven by a collague that the soul is generated at the very instance when its body is assembled. The capacities of the soul grow all the while when it is connected with the body (Wolff conveniently forgets cases of senility). Thus, Wolff concludes, as the state of the soul after death is a mystery to us, it is reasonable to suppose that it will continue developing and perfecting itself.
This is a good example about what I think the greatest fault in the whole chapter on rational psychology. Wolff already knows the answer he must get – soul must be immaterial, it must be immortal and its life after death must be happy and perfect. The grounds for these conclusions are then discovered afterwards, and no puzzle about the nature of the soul has ever actually existed.
So much then for soul: there's only God to discuss anymore.