Note 32 — Chapter XXIII · THE NEW ACADEMY.

(a) Denial of possibility of formal knowledge. To prove the impossibility of knowledge in general, he appeals sometimes to experience. There is no kind of conviction which does not sometimes deceive us; consequently there is none which guarantees its own truth.32 Going then further into the nature of our notions, he argues, that since notions consist in the change produced on the soul by impressions from without, they must, to be true, not only furnish information as to themselves, but also as to the objects producing them. Now, this is by no means always the case, many notions avowedly giving a false impression of things. Hence the note of truth cannot reside in an impression as such, but only in a true impression.33 It is, however, impossible to distinguish with certainty a true impression from one that is false. For independently of dreams, visions, and the fancies of madmen, in short, of all the unfounded chimeras which force themselves on our notice under the guise of truth,34 it is still undeniable that many false notions closely resemble true ones. The transition, too, from truth to falsehood is so gradual, the interval between the two is occupied by intermediate links so innumerable, and gradations so slight, that they imperceptibly pass one into the other, and it becomes impossible to draw a boundary line between the two opposite spheres.35 Not content with proving this assertion in regard to impressions of the senses, Carneades went on to prove it with regard to general notions based on experience and intellectual conceptions.36 He showed that it is impossible for us to distinguish objects so much alike as one egg is to another; that at a certain distance the painted surface seems raised, and a square tower seems round; that an oar in the water seems broken, and the neck-plumage of a pigeon assumes different colours in the sun; that objects on the shore seem to be moving as we sail by, and so forth;37 in all these cases the same strength of conviction belongs to the false as to the true impressions.38 He showed further that this applies equally to purely intellectual ideas; that many logical difficulties cannot be solved;39 that no absolute distinction can be drawn between much and little, in short between all differences in quantity; and that it is the most natural course in all such cases to follow Chrysippus, and to avoid the dangerous inferences which may be drawn by withholding judgment.40 Arguing from these facts, Carneades concluded at first in regard to impressions of the senses, that there is no such thing as φαντασία καταληπτικὴ in the Stoic sense of the term, in other words, that no perception contains in itself characteristics, by virtue of which its truth may be inferred with certainty.41 This fact being granted, the possibility is in his opinion precluded of there residing in the understanding a standard for the distinction of truth from falsehood. The understanding—and this belief was shared by his opponents—must derive its material from the senses.42 Logic tests the formal accuracy of combinations of thought, but gives no insight into their import.43 Direct proofs of the uncertainty of intellectual convictions are not therefore needed. The same result may also be attained in a more personal way, by raising the question, how individuals obtain their knowledge. He can only be said to know a thing who has formed an opinion respecting it. In the mean time, until he has decided in favour of some definite opinion, he has still no knowledge. And what dependence can be placed on the judgment of one who has no knowledge?44