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, and compilations and treatises of those who differ in dogmas, and are confident that they themselves know the truth that there is in things.