David Hume's Treatise of Human Nature is declared to be a "science of man." It is neither "moral philosophy" (a phrase not for ethics, but rather for what we today call psychology) nor "natural philosophy" (mainly physics, chemistry, and biology.) The former investigates the subject, the latter the object. From the Treatise, I suggest that one could construct a third science whose task is to describe the nature and scope of various transitions between subject and object.
What are such "transitions" here? There are certain subjective phenomena which are resolved within the subject, and thus have little to no import on how objects appear to us. The same can be said of objects, that certain physical relations have no influence over how we think or feel. However, there are certain mental phenomena which influence the way objects appear, and conversely there are certain objects which condition how we think and feel. "Transition" is a term which aims to capture this third domain where neither "subject" nor "object" suffice as the adequate name for the matter inquired.
Hume's Treatise contains a rich series of speculations which can be consistently interpreted under the heading of the science of transitions. The term "man" is here substituted with "transitions," and thus man is defined as this ambiguous movement, symbolized in the figure of Hermes, perhaps. Of these transitions, Hume distinguishes two types: automatic ones, and ones which are to an extent controllable. The former is consistently treated under the rubric "custom" or "habit," while the latter is often presented indirectly and implicitly through various claims.
In this context, Book I of the Treatise, where matters such as geometry, arithmetic, causality, chance, probability, and necessity are discussed, one could imagine a sub-title: "Concerning the Automatic Processes of the Mind as They Affect Objects." But what are those "objects?" Hume is not a direct realist. Objects give rise to "impressions," but these impressions are never directly cognized. For Hume, everything which enter into consciousness are already "ideas," that is, impression converted in a certain way. Ideas which are directly connected with their corresponding impressions are true, while any further subjective manipulation can potentially compromise this initial correspondence and thus the truth of them. Now according to Hume, ideas are by themselves true, that is, sufficiently determined as objects of reflection. On the other hand, there are certain relations which we, as humans, impose upon these ideas, and thus form expectations with regards to how future ideas may turn up. The latter relations are: contiguity, resemblance, and causality.
With the division between impressions and ideas, the "ideas of impressions" and the "ideas of ideas," and the different laws which hold for these different domains, we already see that it is unreasonable and fruitless to try and sort these divisions into "objective" and "subjective" categories. Rather, object and subject often overlap in these cases. An idea of an impression, for example, is not purely subjective, since it is conditioned by an object, but neither is it purely objective, since it is retained even after the object becomes absent from immediate perception. Or, to look at impressions themselves, while it is clear that they are in some sense "copies" of external objects, they too are already "subjectivized" in some sense, ready to be converted into ideas, the latter enjoying a more permanent and free kind of existence.
The distinction between "constant conjunction" and "causality" can also be interpreted along these ambiguous lines. Constant conjunction seems to arise out of our direct perception of objects. These objects are already distinct from each other, and thus are ready to enter our consciousness in their plurality. Now these objects then give rise to ideas. If objects are altered, so are our ideas. At this point, we are now able to explicitly experience the "conjunction" of multiple objects. This conjunction is "constant" just in case there is a high degree of resemblance and repetition going on. Causality, however, is not present in all this. This is because causality is an act of isolating two ideas and declaring that the two form a kind of set. So for example we can have the constant conjunction of {a, b, c, d} and {e, f, g, h} at a certain point: two temporal moments, each containing four objects each distinct from the others. Now to say that this conjunction occurs is uncontroversial. But to claim that, for instance, the presence of {a} is the cause of the effect {f} is already to isolate two elements and to assign an additional determination. On this latter model, the conjunction is "in reality" as follows: {b, c, d} and {e, g, h} are distinct moments in time, each separate from the other, while a third set {a, f} overlaps into both moments and exist in a separate order. This additional determination, however, can only be added after the occurrence of the conjunction {a, b, c, d} and {e, f, g, h}. In this sense, for Hume the origin of the idea of a "necessary connexion" (or for that matter, any causal connection) is subjective.
However, this does not mean that causality has no import on how objects appear to us. Causality is part of "human nature" - we cannot dispense with it without lapsing into some kind of dysfunctional slumber. Although causality is not an impression, and thus not something which exists objectively in the natural world, it does govern the way in which we make decisions. At the close of Part I, Hume introduces the notion of a "general rule" by which we judge the nature of various phenomena, from the real meaning of another person's words to the truth of what is written or represented in a certain historical material. Without the idea of causality, there is no way in which we can make such judgments, and there can be no such general rules. But these rules, which Hume cites from the most ordinary everyday interaction of people, are essential to how humans form societies with each other and with other living beings in nature.
Therefore, causality is for Hume an ambiguous idea, yet its proper use is clear. On the side of science, it must be used with a grain of salt, for it is clearly not part of natural objects in themselves, and thus are not to be included in "objective" accounts of nature. In particular, any claim to the "necessity" of a certain course of objective events has to be seen through skeptical eyes. This is the skeptical Hume with whom we are all familiar. But there is a second side to the story. On the side of ethics, causality plays an essential role in coordinating our decisions and our interventions into nature. Here, the idea of causality is a necessary condition for humans to fulfill their "natural" needs. This is Hume's naturalism with regards to causality. But the proper scope of the idea of causality here concerns human action. As such, it is neither purely "objective" nor "subjective," but rather the idea of causality is deployed in the aforementioned third domain of "transitions." This interpretation already anticipates a certain concept of freedom which can further be constructed out of what Hume has to offer his readers in the Treatise. But more on this on another occasion.
