A.T. Robertson Commentary John 8:13

"The Pharisees therefore said unto him, Thou bearest witness of thyself; thy witness is not true." — John 8:13 (ASV)

Of thyself (περ σεαυτου). This technical objection was according to the rules of evidence among the rabbis. "No man can give witness for himself" (Mishnah, Ketub. 11. 9). Hence, they say, "not true" (ουκ αληθες), not pertinent. "They were still in the region of pedantic rules and external tests." In Joh 5:31 Jesus acknowledged this technical need of further witness outside of his own claims (John 19-30) and proceeded to give it (John 32-47) in the testimony of the Baptist, of the Father, of his works, of the Scriptures, and of Moses in particular.