Is Jesus’ witness about himself true or not?

The two sayings answer different questions. In John 5 Jesus speaks in legal terms: the law required “two witnesses” or three to establish a matter (Deuteronomy 19:15), and no one could be a witness in his own cause, so a man’s word about himself, standing alone, did not count as evidence. ‘True’ there (Greek alēthēs) means ‘valid’, and the BSB translates it so: “My testimony is not valid.” Jesus then calls his other witnesses: John the Baptist, the works the Father gave him to do, the Father himself and the Scriptures (John 5:32–39).

In John 8 the Pharisees throw his words back at him: “Thou bearest record of thyself; thy record is not true” (8:13). Jesus answers that his testimony is true in itself, because he knows where he came from and where he is going (8:14), and that it does not stand alone: “I am one that bear witness of myself, and the Father that sent me beareth witness of me” (8:18), which meets the law’s rule of two witnesses (8:17). In both chapters he makes the same point, that his word is backed by the Father; John Gill explained John 5:31 in this way, with ‘true’ set against what is ‘not valid in law’, not against what is false.

‘Of myself’ as ‘on my own’

Some read John 5:31 in the light of the verse before it, “I can of mine own self do nothing” (5:30): a witness given on his own, apart from the Father, would not be true, but Jesus never speaks apart from the Father. Jamieson, Fausset and Brown take the verse this way.