But wilt thou know? (θελεις δε γνωναι?). "But dost thou wish to know?" Ingressive aorist active infinitive of γινοσκω (come to know). James here introduces a new argument like Romans 13:3.

O vain man (ω ανθρωπε κενε). Goes on with the singular objector and demolishes him. For "empty" (deficient) Paul uses αφρων (fool) in 1. Corinthians 15:36 and just ανθρωπε in Romans 2:1; Romans 9:20.

Barren (αργε). See 2. Peter 1:8 (not idle nor unfruitful) and Matthew 12:36, but Hort urges "inactive" as the idea here, like money with no interest and land with no crops.