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may be compar'd 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 endow'd 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 phænomena of the human mind are deriv'd 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. 'Tis 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 wou'd endeavour to establish; there is always some phænomenon, which is more stubborn, and will not so easily bend to our purpose. We need not be surpriz'd, that this shou'd happen in natural philosophy. The essence and composition of external bodies are so obscure, that we must necessarily, in our reasoning's, or rather conjectures conceding them, involve ourselves in contradictions and absurdities. But as the perceptions of the mind are perfectly known, and I have us'd all imaginable caution in forming conclusions conceding them, I have always hop'd to keep clear of those contradictions, which have attended every other system. Accordingly the difficulty, which I have at present in my eye, is no-wise