A witness for him (martu autwi). As in Acts 1:8 . Of what (wn). Attraction of the accusative relative a to the genitive case of the unexpressed antecedent toutwn. Thou hast seen and heard (ewraka, present perfect active indicative kai hkousa, first aorist active indicative). This subtle change of tense is not preserved in the English. Blass properly cites the perfect ewraka in 1 Corinthians 9:1 as proof of Paul's enduring qualification for the apostleship.