Our nature brings sin with it and all men, "according to the spirit", are thus "born blind".

Continuation of the Holy Gospel according to John
John 9:1-38
At that time, Jesus, passing by, saw a man, who was blind from his birth. And so on.

Homily by St. Augustine, Bishop of Hippo.
44th Tract on John.
Dread and wondrous are all the things which our Lord Jesus Christ did, both His works and His words; the works, because He wrought them; the words, because they are deep. If, therefore, we consider the meaning of this work of His, we see that that man which was blind from his birth was a figure of mankind. This spiritual blindness was the consequence of the sin of the first man, from whom we all inherit by birth, not death only, but depravity also. For if blindness be unbelief, and faith, light, whom, when Christ came, did He find faithful? May, the Apostle who had himself been born of the race of which the Prophets came, saith: We also were by nature children of wrath, even as others. Eph. ii. 3. And if children of wrath, then children also of vengeance, children of damnation, children of hell. And wherefore so by nature, unless it were that the sin of the first man had made all his descendants to be born in sin, in that they partook of his nature? If, then, our nature bring sin with it, all men, according to the spirit, are born blind.