Wikipedia: An Enquiry Concerning Human Understanding is a book by the Scottish empiricist and 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 1739–1740. Hume was disappointed with the reception of the Treatise (it "fell dead-born from the press", as he put it) and so tried again to get his ideas before the public in this Enquiry. Among the changes from the Treatise included a removal of Hume's theories of personal identity.
This book was highly influential. Immanuel Kant points to it as the book which woke him from his self-described "dogmatic slumber".
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 this section, by way of warning, that philosophers with nuanced thoughts will likely be cast aside in favor of those who wield rhetoric (or sophists). 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. 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. (In Locke's terminology, this was known as the division between simple and complex ideas of sense). 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 Lilliputans, whose size has been diminished). In a later chapter, he also mentions the operations of mixing, separating, and dividing.
However, Hume admits that his account has one Achilles Heel: the "missing blue shade" problem. In this thought-experiment, he asks us to imagine a man who has experienced every shade of blue except for one. 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.
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, 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, but does not immediately show what this principle might be.
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.
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. When again it is asked, What is the foundation of all our reasonings and conclusions concerning that relation? it may be replied in one word, experience. But if we still carry on our sifting humor, and ask, What is the foundation of all conclusions from experience? this implies a new question, which may be of more difficult solution and explication." (p. 328) He shows how a satisfying argument for the validity of experience can be based neither on demonstration (since "it implies no contradiction that the course of nature may change", p. 330) nor experience (since that would be a circular argument). Here he is describing what would become known as the problem of induction.
5. Skeptical solution of these doubts (in two parts).
For Hume, we assume that experience tells us something about the world because of habit or custom, which human nature forces us to take seriously. This is also, presumably, the "principle" that organizes the connections between ideas. Indeed, one of the many famous passages of the Enquiry was on the topic of the incorrigibility of human custom. Hume wrote: "The great subverter of Pyrrhonism or the excessive principles of skepticism is action, and employment, and the occupations of common life. These principles may flourish and triumph in the schools; where it is, indeed, difficult, if not impossible, to refute them. But as soon as they leave the shade, and by the presence of the real objects, which actuate our passions and sentiments, are put in opposition to the more powerful principles of our nature, they vanish like smoke, and leave the most determined skeptic in the same condition as other mortals."
In the second part, he provides an account of beliefs.
6. Of probability. This short chapter begins with the notions of probability and chance. For him, "probability" means a higher chance of occurring, and brings about a higher degree of subjective expectation in the viewer. By "chance", he means all those particular comprehensible events which the viewer considers possible in accord with their experience. However, further experience takes these equal chances, and forces the imagination to observe that certain chances arise more frequently than others. These gentle forces upon the imagination cause the viewer to have strong beliefs in outcomes. This effect may be understood as another case of custom or habit taking past experience and using it to predict the future.
Applied epistemology
7. Of the idea of necessary connection (in two parts).
By "necessary connection", Hume means the power or force which necessarily ties one idea to another. He rejects the notion that any sensible qualities are necessarily conjoined, since that would mean we could know something prior to experience. He also rejects the idea that volitions or impulses of the will may be necessarily connected to the actions they produce, since (among other reasons) we have no immediate knowledge of the powers which allow an impulse of volition to create an action. He produces his solution to the dilemma: that the idea of a necessary connection arises out of observation of constant conjunction of certain impressions across many instances.
He then produces three formulations of causation.
8. Of liberty and necessity (in two parts). Here Hume tackles the problem of free will, espousing a broadly compatibilist position.
9. Of the reason of animals (comparable to man). Hume insists that the conclusions of the Enquiry will be very powerful if they can be shown to apply to animals and not just humans. He believed that animals were able to infer the relation between cause and effect in the same way that humans can. (His views can be likened to those of behaviorism in 20th century psychology. However, testing on animals such as cats have concluded that they do not possess any faculty which allow their minds to grasp an insight into cause and effect.) He also notes that this "inferential" ability that animals have is not through reason, but custom alone. He concludes that there is an innate faculty of instincts which both beasts and humans share, namely, the ability to reason experimentally (through custom). Nevertheless, he admits, humans and animals differ in mental faculties in a number of ways, including: differences in memory and attention, inferential abilities, ability to make deductions in a long chain, ability to grasp ideas more or less clearly, the human capacity to worry about conflating unrelated circumstances, a sagely prudence which arrests generalizations, a capacity for a greater inner library of analogies to reason with, an ability to detach oneself and scrap one's own biases, and an ability to converse (and thus gain from the experience of others' testimonies).
10. Of miracles (in two parts). The next topic which Hume strives to give treatment is that of the reliability of human testimony, and to the extent that testimony plays a part in epistemology. This was not an idle concern for Hume. Depending on its outcome, the entire treatment would give the epistemologist a degree of certitude in the treatment of miracles.
True to his empirical thesis, Hume tells the reader that, though testimony does have some force, it is never quite as powerful as the direct evidence of the senses. That said, he provides some reasons why we may have a basis for trust in the testimony of persons: because a) human memory can be relatively tenacious; and b) because people are inclined to tell the truth, and ashamed of telling falsities. Needless to say, these reasons are only to be trusted to the extent that they conform to experience.
And there are a number of reasons to be skeptical of human testimony, also based on experience. If a) testimonies conflict one another, b) there are a small number of witnesses, c) the speaker has no integrity, d) the speaker is overly hesitant or bold, or e) the speaker is known to have motives for lying, then the epistemologist has reason to be skeptical of their claims. These criteria are roughly upheld in modern social psychology, under the rubric of the communication-persuasion paradigm.
There is one final criterion that Hume thinks gives us warrant to doubt any given testimony, and that is f) if the propositions being communicated are miraculous. Hume understands a miracle to be any event which contradicts the laws of nature. He argues that the laws of nature have an overwhelming body of evidence behind them, and are so well demonstrated to everyone's experience, that any deviation from those laws necessarily flies in the face of all evidence.
Moreover, he stresses that talk of the miraculous has no surface validity, for four reasons. First, he explains that in all of history there has never been a miracle which was attested to by a wide body of disinterested experts. Second, he notes that human beings delight in a sense of wonder, and this provides a villain with an opportunity to manipulate others. Third, he thinks that those who hold onto the miraculous have tended towards barbarism. Finally, since testimonies tend to conflict with one another when it comes to the miraculous -- that is, one man's religious miracle may be contradicted by another man's miracle -- any testimony relating to the fantastic is self-denunciating.
Still, Hume takes care to warn that historians are generally to be trusted with confidence, so long as their reports on facts are extensive and uniform. However, he seems to suggest that historians are as fallible at interpreting the facts as humans are. Thus, if every historian were to claim that there was a solar eclipse in the year 1600, then though we might at first naively regard that as in violation of natural laws, we'd come to accept it as a fact. But if every historian were to assert that Queen Victoria was observed walking around happy and healthy after her funeral, and then interpreted that to mean that they had risen from the dead, then we'd have reason to appeal to natural laws in order to dispute their interpretation.
11. Of a particular providence and of a future state. Hume continues his application of epistemology to theology by an extended discussion on heaven and hell. The brunt of this chapter narrates the opinions, not of Hume, but of one of Hume's anonymous friends. His friend argues that, though it is possible to trace a cause from an effect, it is not possible to infer unseen effects from any given cause. The friend insists, then, that even though we might postulate that there is a first cause behind all things -- God -- we can't infer anything about the afterlife, because we don't know anything of the afterlife from experience, and we can't infer it from the existence of God.
Hume offers his friend an objection: if we see an unfinished building, then can't we infer that it has been created by humans with certain intentions, and that it will be finished in the future? His friend concurs, but indicates that there is a relevant disanalogy that we can't pretend to know the contents of the mind of God, while we can know the designs of other humans. Hume seems essentially persuaded by his friend's reasoning.
12. Of the academical or sceptical philosophy (in three parts).
The first section of the last chapter is organized as an outline of various skeptical arguments. The treatment includes the arguments of atheism, Cartesian skepticism, "light" skepticism, and rationalist critiques of empiricism. Hume shows that even light skepticism leads to crushing doubts about the world which - while ultimately are more philosophically justifiable - may only be combated through the non-philosophical adherence to custom or habit. He ends the section with his own reservations towards Cartesian and Lockean epistemologies.
In the second section he returns to the topic of excessive skepticism.
He concludes the volume by setting out the limits of knowledge once and for all. He argues that any metaphysical doctrines which cannot be reduced to an analysis of numbers, or matters of fact, ought to be ignored as sophistry and illusion.