In order to indicate a character of a certain being's species, it is necessary that it be grasped under one concept with other species known to us. But also, the characteristic property (proprietas) by which they differ from each other has to be stated and used as a basis for distinguishing them. – But if we are comparing a kind of being that we know (A) with another kind of being that we do not know (non-A), then how can one expect or demand to indicate a character of the former when the middle term of the comparison (tertium comparationis) is missing to us? – The highest species concept may be that of a terrestrial rational being; however, we shall not be able to name its character because we have no knowledge of non-terrestrial rational beings that would enable us to indicate their characteristic property and so to characterize this terrestrial being among rational beings in general. – It seems, therefore, that the problem of indicating the character of the human species is absolutely insoluble, because the solution would have to be made through experience by means of the comparison of two species of rational being, but experience does not offer us this.

Therefore, in order to assign the human being his class in the system of animate nature, nothing remains for us than to say that he has a character, which he himself creates, in so far as he is capable of perfecting himself according to ends that he himself adopts.