1 Samuel 17:55

Whose son is this youth? Quest. How could David be unknown to Saul, with whom he had lived? 1 Samuel 16:21. Answ. That might well be, for divers reasons, because David was not constantly with him, nor, as it seems, used by him, but upon extraordinary occasions, and desperate fits of melancholy; from which possibly he had been free for a good while, by God's special providence and care for his people Israel, that so he might be capable of governing and protecting them against the Philistines, who watched all opportunities against them, and at last broke forth into an open war. Thus David had been for some considerable time dismissed from Saul's court, and was returned home; and therefore it is not strange, if Saul had for the present forgotten David; for kings, because of the encumbrance of public business, and the multitude of persons who come to them on several occasions, may easily forget some persons; yea, such as have frequently been with them, especially their servants, whom they do not use to observe with so much attention and care as they do others. Add to this, that the distemper of Saul's mind might make him forgetful; and that David might now be much changed, both in his countenance and in his habit, from what he had before; and it is apparent, that the change of habits makes so great a difference, that it oft keeps us from the knowledge of those persons whom in other habits we very well know. Some give this answer, That this was the first time that Saul had seen David; and that David's exploit here recorded was performed before that which is recorded 1 Samuel 17:15, though it be placed after it; but that is confuted by comparing 1 Samuel 18:1. I cannot tell; which is not strange, because Abner's conversation and employment was generally in the camp, when David was at the court; and when Abner was there, he took little notice of a person so much inferior to him as David was.