That which is born of the flesh, is flesh,.... Man by his natural birth, and as he is born according to the flesh of his natural parents, is a mere natural man; that is, he is carnal and corrupt, and cannot discern spiritual things; nor can he, as such, enter into, and inherit the kingdom of God; see 1Co 2:14.
And therefore there is a necessity of his being born again, or of the grace of the Spirit, and of his becoming a spiritual man; and if he was to be, or could be born again of the flesh, or ever so many times enter into his mothers womb, and be born, was it possible, he would still be but a natural and a carnal man, and so unfit for the kingdom of God.
By "flesh" here, is not meant the fleshy part of man, the body, as generated of another fleshy substance; for this is no other than what may be said of brutes; and besides, if this was the sense, "spirit", in the next clause, must mean the soul, whereas one soul is not generated from another: but by flesh is designed, the nature of man; not merely as weak and frail, but as unclean and corrupt, through sin; and which being propagated by natural generation from sinful men, cannot be otherwise; for "who can bring a clean thing out of an unclean?