An Enquiry Concerning Human Understanding

Hume was disappointed with the reception of the Treatise, which "fell dead-born from the press,"[3] as he put it, and so tried again to disseminate his more developed ideas to the public by writing a shorter and more polemical work.

The argument of the Enquiry proceeds by a series of incremental steps, separated into chapters which logically succeed one another.

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.

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).

This seems to pose a serious problem for the empirical account, though Hume brushes it aside as an exceptional case by stating that one may experience a novel idea that itself is derived from combinations of previous impressions.

(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.

According to 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 effect may be understood as another case of custom or habit taking past experience and using it to predict the future.

Unlike his predecessors, Berkeley and Locke, Hume rejects the idea that volitions or impulses of the will may be inferred to necessarily connect to the actions they produce by way of some sense of the power of the will.

(Hume 1974:355-356) He also argues in brief against the idea that causes are mere occasions of the will of some god(s), a view associated with the philosopher Nicolas Malebranche.

(Hume 1974:356-359) Having dispensed with these alternative explanations, he identifies the source of our knowledge of necessary connections as arising out of observation of constant conjunction of certain impressions across many instances.

Hume believes that all disputes on the subject have been merely verbal arguments—that is to say, arguments which are based on a lack of prior agreement on definitions.

He then shows (quite briefly) how determinism and free will are compatible notions, and have no bad consequences on ethics or moral life.

Hume concludes that there is an innate faculty of instincts which both beasts and humans share, namely, the ability to reason experimentally (through custom).

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.

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.

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 shows that even light skepticism leads to crushing doubts about the world which - while they ultimately are philosophically justifiable - may only be combated through the non-philosophical adherence to custom or habit.

If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number?

"[6] The criteria Hume lists in his examination of the validity of human testimony are roughly upheld in modern social psychology, under the rubric of the communication-persuasion paradigm.

Supporting literature includes: the work of social impact theory, which discusses persuasion in part through the number of persons engaging in influence; as well as studies made on the relative influence of communicator credibility in different kinds of persuasion; and examinations of the trustworthiness of the speaker.

Testing on certain animals like cats have concluded that they do not possess any faculty which allow their minds to grasp an insight into cause and effect.

However, it has been shown that some animals, like chimpanzees, were able to generate creative plans of action to achieve their goals, and thus would seem to have a causal insight which transcends mere custom.

[7] Albert Einstein was a great admirer of Hume and remarked in a letter to Moritz Schlick that he had read Hume's book and the works of Ernst Mach "with eagerness and admiration shortly before finding relativity theory" and that "very possibly, I wouldn't have come to the solution without those philosophical studies".