“A rare conjunction of words, but fitted here to confirm against ignorance and doubt” (Bengel). For I know, see on Joh 2:4. The persuasion is not the result of his own reasoning, but of his fellowship in the Lord Jesus. So Rev, for by the Lord, etc.
Unclean (κοινὸν)
Lit., common. In the Levitical sense, as opposed to holy or pure. Compare Mar 7:2, “With defiled (κοιναῖς common), that is to say, with unwashen hands.” See Act 10:14.