"You ask me, '_If the idea of cause and effect is nothing but vicinity_,' (you should have said constant vicinity, or, regular conjunction,) I should be glad to know _whence is that farther idea of causation against which you argue_? This question is pertinent, but I hope I have answered it; we feel, after the constant conjunction, an easy transition from one idea to the other, or a connexion in the imagination; and as it is usual for us to transfer our own feelings to the objects on which they are dependent, we attach the internal sentiment to the external objects. If no single instances of cause and effect appear to have any connexion, but only repeated similar ones, you will find yourself obliged to have recourse to this theory. "I am sorry our correspondence should lead us into these abstract speculations. I have thought, and read, and composed very little on such questions of late. Morals, Politics, and Literature have employed all my time; but still the other topics I must think more curious, important, entertaining, and useful, than any geometry that is deeper than Euclid. If in order to answer the doubts started, new principles of philosophy must be laid, are not these doubts themselves very useful? Are they not preferable to blind, and ignorant assent? I hope I can answer my own doubts; but if I could not, is it to be wondered at? To give myself airs, and speak magnificently, might I not observe, that Columbus did not conquer empires and plant colonies? "If I have not unravelled the knot so well, in those last papers I sent you, as perhaps I did in the former, it has not, I assure you, proceeded from want of good will; but some subjects are easier than others: at some times one is happier in his researches and inquiries than at others. Still I have recourse to the _si quid novisti rectius_; not in order to pay you a compliment, but from a real philosophical doubt and curiosity.[334:1]