 What is fundamental to Hume’s entire empiricist program in the Treatise is his “copy-principle” or the claim that “all our ideas, or weak perceptions, are derived from our impressions, or strong perceptions, and that we can never think of any thing which we have not seen without us, or felt in our minds” (TA, 16–7/ 647). Hume goes on to observe that this “discovery” is of considerable importance “for deciding all controversies concerning ideas”. “Whenever any idea is ambiguous”, he continues, we always have “recourse to the impression, which must render it clear and precise” (TA, 7/648). If we suspect that “any philosophical term has no idea annexed to it”, Hume suggests, we can always ask “from what impression that pretended idea is derived?” (TA, 7/648–9) If no impression can be produced, we must conclude “the term is altogether insignificant” (TA, 7/ 648).