Why did Jesus say his testimony is not valid in John 5:31 and in John 8:14 Jesus says his testimony is valid? [duplicate]

Why did Jesus say his testimony is not valid in John 5:31 and in John 8:14 Jesus says his testimony is valid? John 5:31 – If I bear witness concerning Myself, My testimony is not true. John 8:14 – Jesus answered and said to them, "Ev…