Aquinas employs formal abstraction to secure the possibility of mathematics conceived as a theoretical Aristotelian science. Mathematics is a science that investigates real quantity and it grasps its necessary, universal, and changeless properties by means of formal abstraction. In accord with it the paper is divided into two parts. In the first part Aquinas’s conception of (formal) abstraction is explicated against the background of the Aristotelian theory of science and mathematics. In the second part the problems associated with formal abstraction in mathematics are critically assessed.