He was foolish for thinking it was foolishness. But regardless of what you think about my example, God says the natural man finds truth concerning Him foolishness.
The natural man is one living according to the flesh, that is setting his mind for the time being on the things of the flesh, that is with instinct directing his mind. But the unregenerate man also has a spirit, which he may also be living according to, at times. That man is setting his mind for the time being on the things of the spirit, that is according to spiritual perception, with his spirit directing his mind. Romans 8
"For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit, the things of the Spirit. 6For to be carnally minded is death, but to be spiritually minded is life and peace. 7Because the carnal mind is enmity against God; for it is not subject to the law of God, nor indeed can be. 8So then, those who are in the flesh cannot please God."
It is unbiblical to assume that the unregenerate man is always setting his mind on the the flesh, directed by instinct and therefore, being carnal, so that he can never perceive anything with his spirit directing his mind.