After these matters, then, he thinks that he can make us acquainted in a few words with the questions regarding the nature of evil, which have been variously discussed in many important treatises, and which have received very opposite explanations. His words are: “There neither were formerly, nor are there now, nor will there be again, more or fewer evils in the world (than have always been). For the nature of all things is one and the same, and the generation of evils is always the same.”
He seems to have paraphrased these words from the discussions in the Theætetus, where Plato makes Socrates say: “It is neither possible for evils to disappear from among men, nor for them to become established among the gods,” and so on. But he appears to me not to have understood Plato correctly, although professing to include all truth in this one treatise, and giving to his own book against us the title of A True Discourse. For the language in the Timæus, where it is said, “When the gods purify the earth with water,” shows that the earth, when purified with water, contains less evil than it did before its purification.
And this assertion, that there at one time were fewer evils in the world, is one which we make, in harmony with the opinion of Plato, because of the language in the Theætetus, where he says that “evils cannot disappear from among men.”
Source: Against Celsus (New Advent)