This work is an investigation of the theory concerning first principles of practical reason in the work of Thomas Aquinas.In the heart of this theory is the term 'synderesis', which content was built in the discussions of medieval philosophy and theology, from its mention in the Gloss of Jerome on Ezekiel.This term indicates a concept that presents a new character in the moral action theory in comparison with aristotelian ethics, since Thomas Aquinas understands it as the habit of moral first principles, equivalent to Aristotle's habit of theoretical first principles (intellectus principiorum).Thus, the purpose is to understand how the concept of synderesis is developed within moral philosophy of Thomas Aquinas, aiming to individuate the conceptual structure in which it is found and is related with other important terms of his moral philosophy as conscience, natural law and prudence.