Why was Issac refereed to as Abraham's 'only' son in genesis 22?

Abraham had another son named Ishmael by Hagar, a slave girl. When Abraham proved to god that he was faithful to the end, after he was willing to sacrifice Issac, God refereed to Issac as being Abraham's 'only son'. (Genesis 22:12)  However, Abraham had another son in Genesis 16 named Ishmael by Hagar, a slave girl.  Even though Hagar was not his wife, Ismael was still his son--right?

Genesis 16:12

ESV - ¹² He shall be a wild donkey of a man, his hand against everyone and everyone's hand against him, and he shall dwell over against all his kinsmen.

Clarify • Share • Report • Asked December 04 2014 • tom w Supporter