9 He Himself explains this also, and shows us His meaning when He said, “He that has heard and learned of the Father comes unto me.” He immediately subjoined what we were able to conceive: “Not that any man has seen the Father, save he who is of God, he has seen the Father.” What is that which He says? I have seen the Father, you have not seen the Father; and yet ye come not unto me unless you are drawn by the Father. And what is it for you to be drawn by the Father but to learn of the Father? What is to learn of the Father but to hear of the Father? What is to hear of the Father but to hear the Word of the Father— that is, to hear me? In case, therefore, when I say to you, “Every man that has heard and learned of the Father,” you should say within yourselves, But we have never seen the Father, how could we learn of the Father? Hear from myself: “Not that any man has seen the Father, save He who is of God, He has seen the Father.” I know the Father, I am from Him; but in that manner in which the Word is from Him where the Word is, not that which sounds and passes away, but that which remains with the speaker and attracts the hearer.
Source: Tractates on the Gospel of John (New Advent)