HUME'S A TREATISE OF HUMAN NATURE constitutes a philosophical anthropology quite different from a philosophy of (self-)consciousness or of the subject. According to Hume, the Self or Subject is itself a product of human nature, that is, of the workings of a structured set of principles which explains all typically human phenomena. On the same basis, Hume discusses all "moral" subjects, such as science, morality and politics (including economics), art and religion as well as the different reflections about all these such as philosophy, criticism, political theory, history, theology, and so forth. The Treatise therefore is the study of the (de facto) possibility conditions of what is (typically) human. The principles of human nature are not primarily reason, or free will and reason, but the heart, (1) that is, a combination of principles concerning the imagination and the sensibility or the emotions, all of them irrevocably socially determined.
In terms of cognition, humans clearly surpass animals in complexity, even though animals are quite capable of transcending what is merely given at any moment. Already at the level of sensibility, there is--as Kant would also say--a synthesizing activity present in both humans and animals which transforms the mere givenness of similar data into a perception of similarity, and the mere givenness of data appearing next to and after one another into a perception of spatial proximity and temporal succession. The capacity to anticipate things that are not yet there or to postulate things that are not present (or no longer present) is extremely important for survival. This capacity depends on the ability to establish concrete causal links and on the expectations resulting from them. In this way, the domain of reality--what we take into account in our beliefs and actions--expands into a concrete past and a concrete future. The three "natural" relations ensure that what is given is not simply a mere multiplicity but forms an integrated totality of connections, that is, a world, indeed a world not limited to the present. (2) Hume devotes relatively little attention to the astonishing capacities for establishing spatio-temporal connections and connections of resemblance. He does not dwell on what everybody already agreed upon and which was only a subsidiary point as to his real topic: knowledge and probability.
As is well known, his interest is directed more specifically to the relation of causality. (3) He asks himself what might be the basis of this transcendence of the given. (4) Contrary to what philosophers had previously thought, that basis is not the ratio, it is the factual psychological constitution of the human mind. The capacity to establish causal connections is of a piece with our ability to form beliefs or expectations concerning the necessity of these connections. The beliefs are of such a nature that what counts for us is not only what is directly present but also what is believed to belong to the past or the future. Belief is nothing other than a "lively idea": an imaginative activity in which what is not present is taken equally seriously (equally "lively") as what is present. (5)
The principle that permits us to form expectations about the future and to take the past into account is called "reason" by Hume, and it should not be confused with Descartes's ratio since it is also at work in beings that are said to lack reason, such as animals, children, and insane persons. It is a kind of "animal reason," "a wonderful and unintelligible instinct in our souls." (6) In human beings, however, this natural or animal reason is transcended in two ways, discussed successively in book 1, part 3, "Of knowledge and probability" (that is, on the different forms of science), and part 4, "Of the sceptical and other systems of philosophy" (that is, on philosophy), which successively form a kind of nontranscendental analytic and dialectic.
In fact, these transcendences of animal reason cannot be explained exhaustively on the basis of an investigation of the understanding or of the human ability to form and to link together various sorts of ideas. …