A.T. Robertson Commentary James 1:23

"For if any one is a hearer of the word and not a doer, he is like unto a man beholding his natural face in a mirror:" — James 1:23 (ASV)

And not a doer (κα ου ποιητης). Condition of first class, assumed as true, and ου (rather than μη) contrasts ποιητης with ακροατης.

Unto a man beholding (ανδρ κατανοουντ). Associative instrumental case after εοικεν as in 1:6. Note ανδρ as in 1:8 in contrast with γυναικ (woman), not ανθρωπω (general term for man). Present active participle of κατανοεω to put the mind down on (κατα, νους), to consider attentively, to take note of, as in verse 24 (κατενοησεν).

His natural face (το προσωπον της γενεσεως αυτου). "The face of his birth" (origin, lineage, nativity). For this use of γενεσις see 3:6; Matthew 1:1,18; Luke 1:13.

In a mirror (εν εσοπτρω). Old word (from εισ, οπτω) in N.T. only here and 1 Corinthians 13:12. The mirrors of the ancients were not of glass, but of polished metal (of silver or usually of copper and tin). See κατοπτριζομα in 2 Corinthians 3:18.