An Enquiry concerning Human Understanding is a book by the Scottish empiricist philosopher David Hume, published in 1748. It was a simplification of an earlier effort, Hume's A Treatise of Human Nature, published anonymously in London in 1739–40. Hume was disappointed with the reception of the Treatise, which "fell stillborn from the press," as he put it, and so tried again to disseminate his ideas to the public by writing a shorter and more polemical work.
The end product of his labours was the Enquiry. The Enquiry dispensed with much of the material from the Treatise, in favor of clarifying and emphasizing its most important aspects. For example, Hume's views on personal identity, do not appear. However, more vital propositions, such as Hume's argument for the role of habit in a theory of knowledge, are retained.
This book has proven highly influential, both in the years that would immediately follow and today. Immanuel Kant points to it as the book which woke him from his self-described "dogmatic slumber"  The Enquiry is widely regarded as a classic in modern philosophical literature.
The argument of the Enquiry proceeds by a series of incremental steps, separated into chapters which logically succeed one another. After expounding his epistemology, Hume explains how to apply his principles to specific topics.
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 this section, 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 may be either the product of mere sensation, or of the imagination working in conjunction with sensation. 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 substitution of 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 Lilliputians, 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)
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