Mullā Ṣadrā explains self-knowledge through the notion of knowledge-by-presence, which refers to the immediate presence of the known before the knower. A puzzling component of this view is his idea that knower and known have a relationship of unity with one another. Reflection on Sydney Shoemaker's account of self-knowledge can help us uncover Ṣadrā's motivation for this puzzling idea. We show that Ṣadrā was motivated by his awareness of the concept of self-blindness, a notion introduced into contemporary philosophy by Shoemaker.