A.T. Robertson Commentary James 2:20

"But wilt thou know, O vain man, that faith apart from works is barren?" — James 2:20 (ASV)

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 Ro 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 Ro 2:1; 9:20.

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