After assuming that some things were foretold which are impossible in themselves, and inconsistent with the character of God, he says: “If these things were predicted of the Most High God, are we bound to believe them of God simply because they were predicted?” And thus he thinks he proves, that although the prophets may have foretold truly such things of the Son of God, yet it is impossible for us to believe in those prophecies declaring that He would do or suffer such things.
To this our answer is that the supposition is absurd, for it combines two lines of reasoning which are opposed to each other, and therefore mutually destructive. This may be shown as follows. The one argument is: “If any true prophets of the Most High say that God will become a slave, or suffer sickness, or die, these things will come to God; for it is impossible that the prophets of the great God should utter lies.” The other is: “If even true prophets of the Most High God say that these same things shall come to pass, seeing that these things foretold are by the nature of things impossible, the prophecies are not true, and therefore those things which have been foretold will not happen to God.”
When, then, we find two processes of reasoning in both of which the major premiss is the same, leading to two contradictory conclusions, we use the form of argument called “the theorem of two propositions,” to prove that the major premiss is false, which in the case before us is this, “that the prophets have foretold that the great God should become a slave, suffer sickness, or die.” We conclude, then, that the prophets never foretold such things; and the argument is formally expressed as follows: 1st, Of two things, if the first is true, the second is true; 2d, if the first is true, the second is not true, therefore the first is not true.
The concrete example which the Stoics give to illustrate this form of argument is the following: 1st, If you know that you are dead, you are dead; 2d, if you know that you are dead, you are not dead. And the conclusion is— “you do not know that you are dead.” These propositions are worked out as follows: If you know that you are dead, that which you know is certain; therefore you are dead. Again, if you know that you are dead, your death is an object of knowledge; but as the dead know nothing, your knowing this proves that you are not dead. Accordingly, by joining the two arguments together, you arrive at the conclusion— “you do not know that you are dead.” Now the hypothesis of Celsus which we have given above is much of the same kind.
Source: Against Celsus (New Advent)