My research aims to develop a theory of precedent based on Thomas Aquinas’s conception of practical reason. To achieve this, my work focuses on two requirements that every precedent theory must meet, namely elucidating how legal officials follow and distinguish precedents. These practices (hereafter ‘fundamental precedential practices’) must be sufficiently explained; otherwise, it is not possible to understand the nature of the authoritative and conduct-guiding character of law. The first part of my inquiry explains and analyses three major contemporary theories of precedent and the current rule-based model of legal decision-making. The first theory understands precedent as a rule and is defended by Larry Alexander, Frederick Schauer, and Joseph Raz. I call it the rule-based theory of precedent. The second one argues that precedent is an example of behaviour and is defended by Barbara Levenbook. I call it the exemplary theory of precedent. The third one sustains that precedent is a set of reasons and is defended by Grant Lamond. I call it the reason-based theory of precedent. My analysis of these theories and of the contemporary rule-based model of legal decision-making concludes that they cannot explain sufficiently our fundamental precedential practices. Based on Aquinas’s conception of practical reason, the theory of precedent that I propose provides a sufficient account of our fundamental precedential practices and discloses their real nature by explaining: (1) how practical reason determines the salient legal facts of a case and (2) the process of legal decision-making. This theory of precedent advances a Thomistic account of the relatively unexplored notion of legal salience in jurisprudential literature. It also shows how relevant Aquinas's philosophy of action and moral psychology is to his ideas about practical reason, morality, legal decision-making, and law.