The causes productive of scepticism are two things principally. One is the changefulness and instability of the human mind, whose nature it is to generate dissent, either that of one with another, or that of people with themselves. And the second is the discrepancy which is in things; which, as to be expected, is calculated to be productive of scepticism.
For, being unable either to believe in all views, on account of their conflicting nature; or to disbelieve all, because that which says that all are untrustworthy is included in the number of those that are so; or to believe some and disbelieve others on account of the equipoise, we are led to scepticism.
But among the principal causes of scepticism is the instability of the mind, which is productive of dissent. And dissent is the proximate cause of doubt. Whence life is full of tribunals and councils; and, in fine, of selection in what is said to be good and bad; which are the signs of a mind in doubt, and halting through feebleness on account of conflicting matters. And there are libraries full of books, 3709 and compilations and treatises of those who differ in dogmas, and are confident that they themselves know the truth that there is in things.
[The Alexandrians must have recognised this as an ad hominem remark. But see Eccles. xii. 12.]