4. And of such absurdity the cause is this—the supposition of the pre-existence of souls for the first principle of such doctrine leads on the argument by consequence to the next and adjacent stage, until it astonishes us by reaching this point. For if the soul, being severed from the more exalted state by some wickedness after having once, as they say, tasted corporeal life, again becomes a man, and if the life in the flesh is, as may be presumed, acknowledged to be, in comparison with the eternal and incorporeal life, more subject to passion, it naturally follows that that which comes to be in a life such as to contain more occasions of sin, is both placed in a region of greater wickedness and rendered more subject to passion than before (now passion in the human soul is a conformity to the likeness of the irrational); and that being brought into close connection with this, it descends to the brute nature: and that when it has once set out on its way through wickedness, it does not cease its advance towards evil even when found in an irrational condition: for a halt in evil is the beginning of the impulse towards virtue, and in irrational creatures virtue does not exist. Thus it will of necessity be continually changed for the worse, always proceeding to what is more degraded and always finding out what is worse than the nature in which it is: and just as the sensible nature is lower than the rational, so too there is a descent from this to the insensible.