The force of the Greek particle (oun) which introduces this verse is implied by the context itself. The theme of light is now dropped, and it is not resumed again until 9.5. The real heart of this chapter is the question regarding the authority of Jesus.

Now does not actually appear in the Greek text. Good News Translation uses it as a translational technique to emphasize the pronoun you, which is emphatic in the Greek text. Now should not be understood here in a temporal sense.

You are testifying on your own behalf may be rendered, in this particular context, “you are speaking about yourself only on the basis of your own authority” or “you are just talking about yourself without anyone to confirm what you say.” (On the verb testifying, see 1.7.)

What you say proves nothing translates the same expression used in 5.31 (what I say is not to be accepted as real proof), except for the substitution of the pronoun you for I. New English Bible (“your testimony is not valid”) is representative of a number of translations. The adjective “valid” appears in the Revised Standard Version as “true.” It is the same word translated honest in Good News Translation at 7.18. What you say proves nothing may be rendered in some instances “what you say has no value” or “what you say does not convince anyone” or “… cannot convince anyone.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .