Just like Wolffian tradition in general, Darjes distinguishes between empirical study of human personality – the recounting of what we can observe in ourselves – and rational study of it – explanation of these observations. The important characteristic in humanity, Darjes says, is that in addition to body, human person must have as its constituting element some spontaneous entity. He considers for the moment the possibility that a person would consist of more than one spontaneous entity, but finally notes that there is no reason to assume it.
Darjes goes on to develop further common characteristics of this spontaneous entity, based on its essence. First of all, it is an entity, and as such, it can be regarded as a possibility – it does not involve any contradiction – but also as actually existing – is perpetuates and is a substrate for properties. Furthermore, as an entity it must necessarily act.
Secondly, the spontaneous entity constituting one part of human is a simple entity. This means that it cannot be divided into any constituents. This implies, according to Darjes, that this element of humanity cannot be destroyed in the same manner as its body can. Thirdly, this entity acts spontaneously. In other words, it controls its natural conatus toward acting and regulates it according to its own perceptions on what is good or bad.
Darjes notes that the spontaneous entity every human being has two different aspects. Firstly, it is an animal soul, which interacts with body and thus represents things with the inferior cognitive faculty. Secondly, it is a spirit, which is a connected to a nexus of truths and thus represents things with the superior cognitive faculty. Human soul is thus a rational animal, combining features of both animal soul and spirit. Souls in general can then be classified into mere animal souls, rational animal souls and pure spirits. Still, Darjes thinks that these three classes are not completely distinct, but what once was a mere soul and not a spirit could develop into a real spirit.
Animal souls are then characterised by the inferior cognitive faculty. In other words, this animal soul – or just soul – cognises things through the medium of external sensations, which must be explicable through previous external sensations. A mere soul requires new external sensations to get new cognitions, and if the flow of sensations stops, soul effectively dies. That is, the entity that is the soul can well go on existing in another form, but it wouldn't anymore be a mere animal soul.
Spirits, on the other hand, are not intrinsically connected to sensations. That is, even if spirit does not sense anything, it might still produce new representations from its old representations through conceptualising intellect and reasoning. Thus, cessation of sensations does not mean death of a spirit. Furthermore, spirits, Darjes says, are not just spontaneous, but their actions are based on reasoned decisions – in other words, Darjes concludes, spirits are free.
Rational soul, such as that of a human being, is then both an animal soul and a spirit. As a soul, rational soul is dependent on sensations, while as a spirit it should not be dependent on sensations or it should be able to have cognitions without sensations. Still, in another sense rational soul, even as a spirit, is not completely independent of sensations, because sensations or in general changes in the body might hinder the use of conceptual faculties of rational soul. An obvious question is why rational soul needs this connection with the body and sensations, when these just seem to drag it down and restrict it. Darjes suggests as an answer that we require sensations as the original source of cognitions. Indeed, he considers it probable that only God would not require sensations for its cognition, while all finite spirits are finite just because of this dependence on external sensations.
As a spirit, human soul might still exist separately from its body and is thus practically immortal, Darjes notes, although it might still be annihilated. A more problematic question, according to Darjes, is what was the state of human soul before its connection with its body. Darjes notes that the final explanation of this connection must go back to God, but recounts three possible options. Firstly, one might think soul is created out of the souls of its parents. Darjes quickly discards this option, because it would make sense only if soul would be a complex entity.
Secondly, soul might be created by God at the very same moment as the body comes into existence, or thirdly, soul might have lived before the birth of the body. Darjes admits that both options are possible, but leans more clearly to the side of pre-existence. His argumentation is based on observation of human semen, which contains, of course, small organic bodies – this, Darjes insists, is sufficient evidence for the pre-existence of human soul. Clearly, soul in this pre-existent state would not have similar cognitions as us, because the corpuscles of the semen could not sustain human life.