Written by Bachya ibn Pekuda
Trust is peace of mind of the one who trusts. That one relies in his heart that the one he trusts in will do what is good and proper for him on the matter he has trusted him with, according to his ability and his understanding of what will further his good.
But the main factor, which leads one to trust in the one trusted, and without which trust cannot exist, is for one’s heart to be confident that the one trusted will keep his word and will do what he pledged, and that he will also think to do what is good for him even on what he did not pledge out of pure generosity and kindness (this will be explained).