 All of our ideas and beliefs, Mill holds, have their origins in sense impressions. Ideas or sensations which are either (i) “similar” or (ii) “frequently experienced (or thought of) either simultaneously or in immediate succession”, come to be thought of together, and are eventually inseparably bound together in our mind, forming more complex ideas (System, VIII: 852; see Wilson 1990). The theory aims to derive even our most abstract ideas from experience—“Place, Extension, Substance, Cause, and the rest, are conceptions put together out of ideas of sensation by the known laws of association” (Examination, IX: 9)—and thereby undermine the intuitionist’s claim that such ideas are a priori.