|Hume, David (1748 / 2007). An Enquiry concerning Human Understanding. Oxford: Oxford University Press. S.23.|
 Though it be too obvious to escape observation, that different ideas are connected together; I do not find, that any philosopher has attempted to enumerate or class all the principles of association; a subject, however, that seems worthy of curiosity. To me, there appear to be only three principles of connexion among ideas, namely, Resemblance, Contiguity in time or place, and Cause or Effect.