Jamieson, Fausset and Brown on Galatians 4:23

23. after the flesh--born according to the usual course of nature: in contrast to Isaac, who was born "by virtue of the promise" (so the Greek), as the efficient cause of Sarah's becoming pregnant out of the course of nature (Ro 4:19). Abraham was to lay aside all confidence in the flesh (after which Ishmael was born), and to live by faith alone in the promise (according to which Isaac was miraculously born, contrary to all calculations of flesh and blood).