The principal points in the philosophy of Arcesilaus are preserved by Cicero in his _Academics Quæstiones_, but Sextus Empiricus is more valuable as an authority, for he is more thorough, definite, philosophic and systematic. _a._ This philosophy is specially known to us as being a dialectic directed against Stoicism, with which Arcesilaus had much to do, and its result, as far as its main principles are concerned, is expressed thus: “The wise man must restrain his approbation and assent.”[170] This principle was called ἐποχή, and it is the same as that of the Sceptics; on the other hand this expression is connected with the principle of the Stoics as follows. Because to Stoic philosophy truth consists in the fact that thought declares some content of existence to be its own, and the conception as comprehended gives its approbation to this content, the content of our conceptions, principles and thoughts undoubtedly appears to be different from thought, and the union of the two, which is the concrete, only arises by means of some determinate content being taken up into the form of thought and thus being expressed as the truth. But Arcesilaus saw this consequence, and his saying that approbation most be withheld is thus as much as saying that by thus taking up the content no truth comes to pass, but only phenomenon; and this is true, because, as Arcesilaus puts it, conception and thought likewise remain apart. Arcesilaus has certainly unthinkingly allowed that this content united to consciousness is a concrete such as was indicated, only he has asserted that this connection merely gives a perception with a good ground, and not what he calls truth. This is called probability, but not quite appropriately; it is a universal set forth through the form of thought, and is only formal, having no absolute truth. Sextus (Pyrrh. Hyp. I. c. 33, § 233) puts this plainly in saying that “Arcesilaus has declared the withholding of approbation in relation to parts, to be a good, but the assenting to parts to be an evil,” because the assent only concerns _parts_. That is, if thought is to be retained as a universal, it cannot come to be a criterion; and that is the meaning of Arcesilaus when he asks that the wise man should remain at the universal, and not go on to the determinate as if this determinate were the truth.