Here in John 5, Jesus provides a key principle that will help us. In verse 31, He defends Himself against accusations of blasphemy and Sabbath-breaking. He says,
His statement may seem unusual because, for us, what He says is the final word on any matter. He is the Faithful and True Witness (Revelation 3:14). For us, His testimony outweighs everything. But the ESV suggests a word that helps to make His meaning clear. It says, If I alone bear witness about [M]yself, [M]y testimony is not true. In other words, within the context, if His witness about Himself were the only witness, it would not have been accepted by those who were accusing Him. On a human level, Jesus acknowledges that something additional was required for even His own witness to be a valid testimony.
The basis for Christ's statement is the law He gave about establishing significant matters by the testimony of two or three witnesses (Deuteronomy 17:6; 19:15; II Corinthians 13:1). In the rest of John 5, down through verse 46, He then provides additional witnesses of Himself to prove that He was neither blaspheming, nor guilty of violating the Fourth Commandment.
Jesus thus sets the pattern for us in establishing or judging critical matters. This must be our approach when it comes to biblical teaching: We must find multiple witnesses of Scripture to keep away from disputes over doubtful things (Romans 14:1). Jesus teaches that a single testimony is invalid. He says a couple of chapters later, He who speaks from himself seeks his own glory (John 7:18). An untested interpretation is the same as leaning on our own understanding, or being wise in our own eyes (Proverbs 3:5-7; 26:12; Isaiah 5:21).
But the most important reason not to base a doctrine on the definition of a word is that the definition only constitutes a single witness. Such a solitary testimony is not valid, as Jesus Himself establishes.
