Galatians 4:23

Was born after the flesh - Ishmael was born according to the course of nature, his parents being both of a proper age, so that there was nothing uncommon or supernatural in his birth: this is the proper meaning of the apostle's κατα σαρκα , after or according to the flesh, and answers to the Hebrew phrase, בשר דרך על al derec basar , according to the manner of the flesh, i.e. naturally, according to the common process of nature.

By promise - Both Abraham and Sarah had passed that age in which the procreation of children was possible on natural principles. The birth, therefore, of Isaac was supernatural; it was the effect of an especial promise of God; and it was only on the ground of that promise that it was either credible or possible.