70 The heretics cannot deny that the Lord used these words to signify the mystery His birth, but they attempt to escape from them by referring them to a harmony of will. They make the unity of God the Father and God the Son not one of divinity, but merely of will: as if the divine teaching were poor in expression and the Lord could not have said, I and the Father are one in will; or as if those words could have the same meaning as I and the Father are one; or as if He meant, He that has seen My will, has seen the will of My Father also, but, being unskilled statement, tried to express that idea in the words, He that has seen Me has seen the Father also: or as if the divine vocabulary did not contain the terms, The will of My Father is in Me, and My will is in the Father, but this thought could be expressed by I in the Father and the Father in Me.
All this is nauseous and irreverent nonsense; common sense condemns the judgment of such silly fancies, as that the Lord could not say what He wanted, or did not say what He said. True, we find Him speaking in parables and allegories, but it is a different thing to strengthen one's words with illustrations, or satisfy the dignity of the subject with the help of suggestive proverbs, or adapt one's language to the needs of the moment. But this passage concerning the unity, of which we are speaking, does not allow us to look for the meaning outside the plain sound of the words.
If Father and Son are one, in the sense that They are one in will, and if separable natures cannot be one in will, because their diversity of kind and nature must draw them into diversities of will and judgment, how can They be one in will, not being one in knowledge? There can be no unity of will between ignorance and knowledge. Omniscience and nescience are opposites, and opposites cannot be of the same will.
Source: On the Trinity (New Advent)