What does John 8:13 mean?

Jesus has just made an outrageous claim about Himself: "I am the light of the world." Surrounded by lamps made from the garments of priests, as part of the Feast of Booths, Jesus has laid claim to absolute moral and spiritual truth. His phrasing echoed God's own words to Moses: "I AM" (Exodus 3:14). This sparks a dialogue which escalates until the Pharisees are enraged enough to attempt to kill Jesus then and there (John 8:59).

Earlier in His ministry, Jesus was challenged by religious leaders for proof of His claims. There, Jesus agreed evidence was necessary; it was improper to simply take someone's word when they made bold claims. In that exchange, not long after healing a man crippled for decades, Jesus offered three separate lines of evidence supporting His ministry (John 5:30–47). In that context, and for those issues, Jesus agreed that other evidence was not only available, but necessary. This is the same basic argument being made here, this time by the Pharisees.

However, the information being discussed here is not the kind for which Jesus offered human evidence. Those were issues such as eyewitness to miracles and the content of the Scriptures. Here, as Jesus will respond in the next verse, only one person has ever actually seen the truths being claimed, so only that person can speak of them.