No German metaphysician at the middle of 18th century could avoid the question what happens between soul or spirit and body, and Darjes is no exception to this trend. He is quite clear that soul and body are in some measure harmonious, that is, from occurrings in body it is possible to know something about occurrings in soul and vice versa. He is also quite explicit that this harmony is not complete, in other words, that there are actions in body or soul that are not reflected in the actions of the other.
Wolffian school recognised three different solutions to the mind-body problem: firstly, the assumption that soul and body had some real causal influence to one another, secondly, the occasionalism, according to which God bridged the divide between soul and body, and thirdly, the Leibnizian pre-established harmony. Like everyone else in Wolffian school, Darjes does not take occasionalism seriously, because it contradicts the freedom of human actions. Furthermore, like many philosophers of his generation, Darjes does not necessarily respect the traditional Wolffian answer or the pre-established harmony. Indeed, Darjes notes that because harmony of soul and body is to be explained, just assuming the existence of such a harmony is no real answer.
The only respectable option in Darjes' eyes is then the assumption of real interaction between soul and body. Like his wont always, Darjes makes intricate distinctions. He notes that body influences soul in quite a different manner than soul influences body. As a spontaneously active entity, soul cannot be affected by a causal chain originating outside it. Instead, Darjes argues, some movement in the nervous fluid can only remove impediments from the activities of soul and thus give occasion to some sensuous representation. Soul, on the other hand, should have the ability to casually influence nervous fluid and through it whole body.
Darjes concludes his account of rational psychology with few remarks on activities of soul and ways to classify souls and spirits and put them into a hierarchy (it is no wonder that he places soul requiring perceptual content in a lower rung than pure spirit). What we might still point out is his remark that soul cannot be said to be present in a single place in the body, but in the whole body and all its organic parts.