“Then Jesus again spoke to them, saying, “I am the Light of the world; he who follows Me will not walk in the darkness, but will have the Light of life.” 13 So the Pharisees said to Him, “You are testifying about Yourself; Your testimony is not true.” 14 Jesus answered and said to them, “Even if I testify about Myself, My testimony is true, for I know where I came from and where I am going; but you do not know where I come from or where I am going. You judge according to the flesh; I am not judging anyone. 16 But even if I do judge, My judgment is true; for I am not alone in it, but I and the Father who sent Me. 17 Even in your law it has been written that the testimony of two men is true. 18 I am He who testifies about Myself, and the Father who sent Me testifies about Me.”” -John 8:12-18 NASB
Here we see the Son teach that He is not alone, either in His judgements, or His testimony concerning Himself, because the Father was with Him in both of these things. He says “My judgment is true; for I am not alone in it, but I and the Father who sent Me.” Well, if the Father and Son are one and the same person, one and the same rational individual being, then the Son would indeed be alone in His judgement, the Father not in fact being another, but merely a different name or mode or manifestation of Himself. But the Father, we see, is indeed another, and not individually the same, as the Son; or else the Son’s point is nullified, that He is not alone in the judgement.
But let us also observe that the Jews tried to discredit the Lord by saying that, since He testified of Himself, His testimony was not to be accepted. The Son rightly responds, firstly, that His own testimony is reliable; but then, refutes them from this fact: that the law establishes testimony on the witness of two men; and since His Father testifies to Him as well, the Son is not alone, but has a second witness, the Father; and so, having two testifying to the Son’s identity, the Jews ought to have accepted this, even on the basis of the Law’s standard. Now, this entire argument made by the Son here, will be found to be utterly worthless, false, yeah, even deceptive, were the Father and Son not really distinct from one another, if They are not really numerically two, not one. For the whole argument depends on the fact that the Father and Son together are two witnesses, not one; a thing which will be impossible, if They are both the same person, or the same individual being. For one person testifying to himself is but one witness; and even if we wish to indulge the scholastic heresy, of an individual being which is both Father and Son, then we must ask, how does this make for two witnesses, when They are taught to be numerically and individually one and the same? Two modes of one person, or of one individual being, do not make two witnesses, but will rather be found to be one witness, testifying to Himself twice.
We see then, from this passage, that those who make the Son and Father out to be numerically and individually one and the same individual being or person, make the Lord out to be a liar; contradicting His own testimony, that He and the Father are two distinct persons, or individual beings; metaphorically one in the close unity between Them, as many men are said to be one in unity; but numerically and individually two, or else, the Lord’s testimony here, would be false.