But wilt thou know? (theleis de gnōnai̇). “But dost thou wish to know?” Ingressive aorist active infinitive of ginoskō (come to know). James here introduces a new argument like Rom 13:3.
O vain man (ō anthrōpe kene). Goes on with the singular objector and demolishes him. For “empty” (deficient) Paul uses aphrōn (fool) in 1Co 15:36 and just anthrōpe in Rom 2:1; Rom 9:20.
Barren (arge). See 2Pe 1:8 (not idle nor unfruitful) and Mat 12:36, but Hort urges “inactive” as the idea here, like money with no interest and land with no crops.