I applied mine heart to know, and to search, and to seek out wisdom, and the reason of things, and to know the wickedness of folly, even of foolishness and madness:
I applied mine heart to know ... - literally, I turned myself and mine heart to. A phrase special to Ecclesiastes, and appropriate to the penitent turning back to commune with his heart on his past life. Or else, in Hengstenberg's view, in contrast to a superficial search into wisdom.
The reason (of things), х cheshbown (H2808)] - literally, the calculation or right estimation of things: philosophy; ratiocination х logismos (G3053)].
The wickedness of folly. He is now a step further on the path of penitence than Ecclesiastes 1:17; Ecclesiastes 2:12, where "folly" is put without "wickedness" prefixed.
Even of foolishness (and) madness - rather, 'and the foolishness (i:e., the sinful folly, answering to the wickedness in the parallel) of madness, i:e., of man's mad pursuits. Or else, as Hengstenberg translates, 'To know wickedness as folly, and foolishness as madness.'