14 Here is another blasphemy which he has spoken of the Son. “Assuming that the Son knows the Father, it would seem that by this knowledge He can comprehend Him as much as a craftsman can comprehend the rules of his art. And, doubtless, if the Father is in the Son, He is also comprehended by Him in whom He is. But if we mean by comprehension not merely that the knower takes a thing in by perception and insight but that he contains it within himself by virtue of a special faculty; in this sense we cannot say that the Son comprehends the Father. For the Father comprehends all things, and of these the Son is one; therefore, He comprehends the Son.” And to show us reasons why, while the Father comprehends the Son, the Son cannot comprehend the Father, he adds: “the curious reader may inquire whether the Father knows Himself in the same way that the Son knows Him. But if he recalls the words: 'the Father who sent me is greater than I,' he will allow that they must be universally true and will admit that, in knowledge as in everything else, the Father is greater than the Son, and knows Himself more perfectly and immediately than the Son can do.”
Source: Letters (New Advent)