EPISTEMOLOGY thing is signiWed by such a word (which may, of course, be expressed in a sentence of paraphrase, such as ‘A cow is an animal of such-and-such a kind’) The ‘pro’ in ‘prolepsis’ is meant to indicate that a concept of X is not a set of information about X derived from experience, but rather a template by which we recognize in advance whether an individual presented in experience is or is not an X Concepts are not things that have to be proved: they are themselves employed in any proof (D.L 10 33, 38) It remains obscure, in both Epicurus and his followers, whence concepts originate They cannot all be the result of experience, since they provide the means by which we sort sensations, which are the basis of experience But some of them seem to be the result of experience— perhaps misinterpreted experience, like the concept of God (Lucretius 1169–71) Sensations and concepts, for Epicurus, are both ‘evident’ (so too are feelings, but they will be considered in a diVerent context) It is on these evident elements that we must base our beliefs in what is not evident We start with the senses, he said, and then must infer the non-evident by reasoning from their testimony (D.L 10 39) Conjectures and theories are false if the senses bear witness against them (D.L 10 50–1) A conjecture is true if it is conWrmed by the senses; a theory is true if it is not impugned by the senses (S.E., M 213) The latter claim seems surprising: may not more than one incompatible theory be consistent with the evidence? The Epicureans accepted this possibility; thus Lucretius accepts that there may be diVerent explanations of the movements of the stars, just as there may be diVerent hypotheses about the cause of death of a corpse on a slab (6 703–11) In such a case they should all be accepted: each of them is likely to be true in one or other of the many worlds in the universe, even if we not know which is true in our world (5 526–33) Stoic Epistemology The early Stoics shared with the Epicureans a number of assumptions about the nature of knowledge Like them, they believed it must have a dual basis of infallible sense-impressions and primitive and acquired concepts On the topic of concepts they are more informative than the Epicureans, and they 169