  But even when Leibniz accepts the common way of speaking – that is, as if the senses are causally responsible for some ideas – he has arguments against the empiricist claim that the senses are the origin of all ideas. According to Leibniz, while the empiricist position can explain the source of contingent truths, it cannot adequately explain the origin and character of necessary truths. For the senses could never arrive at the universality of any necessary truth; they can, at best, provide us with the means of making a relatively strong induction. Rather, it is the understanding itself, Leibniz claims, which is the source of such truths and which guarantees their very necessity. While we are not aware of all our ideas at any time – a fact demonstrated by the function and role of memory – certain ideas or truths are in our minds as dispositions or tendencies. This is what is meant by an innate idea or an innate truth. Indeed, Leibniz believes that the mind has a “special affinity” for necessary truths. On this subject, Leibniz uses a distinctive metaphor: a piece of marble has veins that indicate or are disposed to indicate shapes that a skillful sculptor can discover and exploit. Similarly, there is a “disposition, an aptitude, a preformation, which determines our soul and brings it about that [necessary truths] are derivable from it” (A VI vi 80/RB 80). 