20 If you still wish to assert that God the Father in solitude said these words to Himself, I can go with you as far as to admit the possibility that He might in solitude have spoken to Himself as if He were conversing with a companion, and that it is credible that He wished the words I have made man after the image of God to be equivalent to I have made man after My own image. But your own confession of faith will refute you. For you have confessed that all things are from the Father, but all through the Son; and the words, Let Us make man, show that the Source from Whom are all things is He Who spoke thus, while God made him after the image of God clearly points to Him through Whom the work was done.
Source: On the Trinity (New Advent)