“If I bear witness of myself, my witness is not true.”
Clearly Jesus did not literally mean that His witness was not true simply because it was His own. He was rather acknowledging that self-testimony was seen as worthless by the Jews from a judicial point of view. This was the strongly held Jewish viewpoint based on the Scriptures. He is therefore stressing that He does not expect them to rely on such self-testimony. Rather there are others who testify of Him. This does not therefore contradict John 8:14, rather it indicates what the response of men will be. Men will say that ‘truth' judicially can only be established by more than one witness. In contrast John 8:14 is saying that essential truth can be established by Him in that He is the One Who truly knows because He is in a position to know, and because of where He comes from, even though it might not be judicially acceptable on earth. The idea there is that by its very nature as heavenly, heavenly truth is more acceptable than earthly truth and requires no further witness.