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The opening of a book of Lucretius’ De Rerum Natura in an illuminated manuscript from the British Library

E P I S T E M O L O G Y

Lucretius, like Aristotle, points out that one sense cannot be corrected

by another with regard to its proper object. But the Epicureans go

further than Aristotle in claiming a sense cannot even correct its

own impressions: each impression is of equal reliability and hence

whatever appears to a sense at any time is true (Lucretius 4. 497–9;

D.L. 10. 31).

By treating all appearances as on a par, instead of grading them in terms

of reliability, Epicureans rule out Aristotle’s method of dealing with con-

Xicting impressions, such as that of a tower that looks round from a

distance but square close up. Instead, they claim that in such a case we

have two equally valid impressions, but impressions of diVerent objects.

Sextus Empiricus explains how Epicurus would deal with the problem, by

invoking his atomistic explanation of sight as an encounter with a stream

of images Xowing from an object of vision.

I would not say that sight is deceived when from a great distance it sees a tower

as small and round, and from nearby as large and square. Rather, it is quite

correct. When what is perceived appears small and so-shaped, it really is small and

shaped like that, because the edges of the images have been rubbed oV as a result of

their journey through the air. And when it appears big and of a diVerent shape,

once again it really is big and of that shape. But the two are not the same.

(M. 7. 208)

Our common impression that these are two glimpses of the same thing,

Epicurus says, is due not to perception but to ‘distorted belief’. He deals in a

similar way with other objections to the infallibility of sensation, such as

dreams and delusions. When Orestes thought he saw the Furies, his sight

was not deceived because there were genuine images present; it was his

mind that erred in taking them as solid bodies (S.E., M. 8. 63). We must

distinguish sharply between a sense-impression (phantastike epibole) and an

accompanying, but distinct, belief (D.L. 10. 51).

Sensations, therefore, the Wrst criteria of truth, in spite of their

infallibility, provide only a rather slender base for the structure of our

knowledge. We need to turn to the second set of criteria, namely concepts.

Epicurus’ word ‘prolepsis’ is often translated ‘preconceptions’, but that

is misleading, partly because it suggests prejudice, partly because it suggests

something that would be expressed by a whole proposition, while most

of the examples we are given are expressed by single words, such as

‘body’, ‘man’, ‘cow’, ‘red’. A concept is a general notion of what kind of

168

E P I S T E M O L O G Y

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.

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