A Treatise of Human Nature by David Hume — Read

Ideas may be compared to the extension and solidity of matter, and impressions, especially reflective ones, to colours, tastes, smells and other sensible qualities. Ideas never admit of a total union, but are endowed with a kind of impenetrability, by which they exclude each other, and are capable of forming a compound by their conjunction, not by their mixture. On the other hand, impressions and passions are susceptible of an entire union; and like colours, may be blended so perfectly together, that each of them may lose itself, and contribute only to vary that uniform impression, which arises from the whole. Some of the most curious phaenomena of the human mind are derived from this property of the passions.

In examining those ingredients, which are capable of uniting with love and hatred, I begin to be sensible, in some measure, of a misfortune, that has attended every system of philosophy, with which the world has been yet acquainted. It is commonly found, that in accounting for the operations of nature by any particular hypothesis; among a number of experiments, that quadrate exactly with the principles we would endeavour to establish; there is always some phenomenon, which is more stubborn, and will not so easily bend to our purpose. We need not be surprized, that this should happen in natural philosophy. The essence and composition of external bodies are so obscure, that we must necessarily, in our reasonings, or rather conjectures concerning them, involve ourselves in contradictions and absurdities. But as the perceptions of the mind are perfectly known, and I have used all imaginable caution in forming conclusions concerning them, I have always hoped to keep clear of those contradictions, which have attended every other system. Accordingly the difficulty, which I have at present in my eye, is nowise contrary to my system; but only departs a little from that simplicity, which has been hitherto its principal force and beauty.