They are all plain... — Because “the secret of the Lord is (only) with them that fear Him “(Psalms 25:14), and God reveals such things unto them by His Spirit (1 Corinthians 2:10), while the “natural man receiveth not the things of the Spirit of God, for they are foolishness unto him” (ibid., Proverbs 8:14).
Related Commentaries of Proverbs 8:9
Proverbs 8:9
9 They are all plain to him that understandeth, and right to them that find knowledge.