An Enquiry Concerning Human Understanding

The Enquiry (1748) is Hume's revision of Book I of the Treatise of Human Nature, rewritten for a general audience and sharper in its conclusions. Its central argument concerns causation. We believe that causes necessitate their effects, but necessity is not found in experience: we observe constant conjunction (one event regularly following another) and form a habit of expectation. The "necessary connection" we think we perceive in the world is a feeling in us, projected outward, not a feature of things themselves.

Section X, "Of Miracles," applies this analysis to religious testimony. A miracle is a violation of the laws of nature. The uniform experience establishing those laws is always stronger evidence than any testimony claiming an exception. No testimony can establish a miracle, because the falsehood of testimony is always more probable than the violation of natural law. This is the sharpest philosophical attack on revealed religion in the tradition.

Hume closes with his famous fork: every meaningful proposition either concerns "relations of ideas" (demonstrable a priori, like mathematics) or "matters of fact" (grounded in experience). Anything that fits neither category, including most of metaphysics and theology, should be "committed to the flames, for it can contain nothing but sophistry and illusion."

The Enquiry is more accessible than the Treatise but not less serious. Kant credited Hume with waking him from dogmatic slumber, and the problem of induction Hume raised, how repeated experience can justify expectation about the future, remains unsolved.