The Scofield Bible Commentary, by Cyrus Ingerson Scofield, , at sacred-texts.com
A "fool" in Scripture is never a mentally deficient person, but rather one arrogant and self-sufficient; one who orders his life as if there were no God. See, for illustration, (Luk 12:16-20). The rich man was not mentally deficient, but he was a "fool" because he supposed that his soul could live on the things in the barn, giving no thought to his eternal wellbeing.
(See Scofield) - (Psa 19:9).