Going by the context you have Philip (vs8) asking Jesus to show him God the Father. Jesus at (vs9) was somewhat saddened by Philip's dullness, and His words imply it.
V10, "Do you not believe that I am in the Father, and the Father is in Me? The words that I say to you I do not speak on My own initiative, but the Father abiding in Me does His works." Vs11, "Believe Me that I am in the Father and the Father is in Me; otherwise believe on account of the works themselves."
What's interesting is the fact that Jesus had talked publicly about this very thing at John 10:7-38, "If I do not the works of My Father do not believe Me; vs38, but if I do them, though you do not believe Me, believe the works, (Why?) that you may know and understand that the Father is in Me, and I am in the Father."
So on what basis or in what sense is the Father in Jesus and Jesus is in the Fater? It starts at John 10:30 when Jesus said, "I and the Father are one." The Jews reacted to this statement at vs31, "The Jews took up stones again to stone Him." At vs32, Jesus ask why? "I showed you many good works from the Father; for which of them are you stoning Me?"
Vs33, "The Jews answered Him, "For a good work we do not stone You, but for blasphemy; and because You, being a man, make Yourself out to be God."
Getting back to John 14, verse 9, Jesus says, "He who has seen Me has seen the Father; how do you say, "Show us the Father?" Jesus is NOT teaching He is God the Father here. The Father has no separate manifestation from the Son. The Son is the only manifestation and revelation of the Father. What is known of the Father is revealed through the Son.
To see the Son is to see the essence of the Father, (John 1:1,18; John 10:30. John 12:45, Colossians 1:15; Hebrews 1:30. Going back to John 10:30 the verse literally says, "I and the Father we are one." It is a neuter number to indicate equality of essence, attributes, design, will, work.
"One" (meaning "one thing," only excluding personal identity. Jesus distinguishes the "I" from these words, thus separating the persons within the Godhead. One asserts their unity of essence or nature as identical. It's already a given that the Father are one in purpose and work.
This is brought out at John 10:25-29 where Jesus explains that the sheep are safe in His hands and in the Father's hands. In short, the mutual indwelling of the Father and the Son are equal.