And now, after these arguments, and others of a similar kind, how can Celsus escape appearing in a ridiculous light, when he imagines that there never has been in the past, nor will be in the future, a greater or less number of evils? For although the nature of all things is one and the same, it does not at all follow that the production of evils is a constant quantity. For although the nature of a certain individual is one and the same, yet his mind, and his reason, and his actions, are not always alike: there being a time when he had not yet attained to reason; and another, when, with the possession of reason, he had become stained with wickedness, and when this increased to a greater or less degree; and again, a time when he devoted himself to virtue, and made greater or less progress therein, attaining sometimes the very summit of perfection, through longer or shorter periods of contemplation. In like manner, we may make the same assertion in a higher degree of the nature of the universe, that although it is one and the same in kind, yet neither do exactly the same things, nor yet things that are similar, occur in it; for we neither have invariably productive nor unproductive seasons, nor yet periods of continuous rain or of drought. And so in the same way, with regard to virtuous souls, there are neither appointed periods of fertility nor of barrenness; and the same is the case with the greater or less spread of evil. And those who desire to investigate all things to the best of their ability, must keep in view this estimate of evils, that their amount is not always the same, owing to the working of a Providence which either preserves earthly things, or purges them by means of floods and conflagrations; and effects this, perhaps, not merely with reference to things on earth, but also to the whole universe of things which stands in need of purification, when the wickedness that is in it has become great.
Source: Against Celsus (New Advent)