Epistemologia Hume

Empirical epistemology
1. Of the different species of philosophy
In the first section of the Enquiry, Hume provides a rough introduction to philosophy as a whole. For Hume, philosophy can be split into two general parts: natural philosophy and the philosophy of human nature (or, as he calls it, "moral philosophy"). The latter investigates both actions and thoughts. He emphasizes in thissection, by way of warning, that philosophers with nuanced thoughts will likely be cast aside in favor of those whose conclusions more intuitively match popular opinion. However, he insists, precision helps art and craft of all kinds, including the craft of philosophy.
2. Of the origin of ideas
Next, Hume discusses the distinction between impressions and ideas. By "impressions", he means sensations,while by "ideas", he means memories and imaginings. According to Hume, the difference between the two is that ideas are less vivacious than impressions. For example, the idea of the taste of an orange is far inferior to the impression (or sensation) of actually eating one. Writing within the tradition of empiricism, he argues that impressions are the source of all ideas.
Hume accepts that ideas maybe either the product of mere sensation, or of the imagination working in conjunction with sensation.[3] According to Hume, the creative faculty makes use of (at least) four mental operations which produce imaginings out of sense-impressions. These operations are compounding (or the addition of one idea onto another, such as a horn on a horse to create a unicorn); transposing (or the substitutionof one part of a thing with the part from another, such as with the body of a man upon a horse to make a centaur); augmenting (as with the case of a giant, whose size has been augmented); and diminishing (as with Lilliputans, whose size has been diminished). (Hume 1974:317) In a later chapter, he also mentions the operations of mixing, separating, and dividing. (Hume 1974:340)

Fig. 1. TheMissing Shade of Blue
However, Hume admits that there is one objection to his account: the problem of "The Missing Shade of Blue". In this thought-experiment, he asks us to imagine a man who has experienced every shade of blue except for one (see Fig. 1). He predicts that this man will be able to divine the color of this particular shade of blue, despite the fact that he has never experienced it.This seems to pose a serious problem for the empirical account, though Hume brushes it aside as an exceptional case. (Hume 1974:319)
3. Of the association of ideas
In this chapter, Hume discusses how thoughts tend to come in sequences, as in trains of thought. He explains that there are at least three kinds of associations between ideas: resemblance, contiguity in space-time, and cause-and-effect.He argues that there must be some universal principle that must account for the various sorts of connections that exist between ideas. However, he does not immediately show what this principle might be. (Hume 1974:320-321)
4. Skeptical doubts concerning the operations of the understanding (in two parts)
In the first part, Hume discusses how the objects of inquiry are either "relations of ideas"or "matters of fact", which is roughly the distinction between analytic and synthetic propositions. The former, he tells the reader, are proved by demonstration, while the latter are given through experience. (Hume 1974:322) In explaining how matters of fact are entirely a product of experience, he dismisses the notion that they may be arrived at through a priori reasoning. For Hume, every effectonly follows its cause arbitrarily—they are entirely distinct from one another. (Hume 1974:324)
In part two, Hume inquires into how anyone can justifiably believe that experience yields any conclusions about the world:
"When it is asked, What is the nature of all our reasonings concerning matter of fact? the proper answer seems to be, that they are founded on the relation of cause and effect....