Jamieson, Fausset and Brown on John 3:6-8

6-8. That which is born, &c.--A great universal proposition; "That which is begotten carries within itself the nature of that which begat it" [Olshausen].

flesh--Not the mere material body, but all that comes into the world by birth, the entire man; yet not humanity simply, but in its corrupted, depraved condition, in complete subjection to the law of the fall (Ro 8:1-9). So that though a man "could enter a second time into his mother's womb and be born," he would be no nearer this "new birth" than before (Job 14:4; Psa 51:5).

is spirit--"partakes of and possesses His spiritual nature."