St. Thomas Aquinas' definition of the conscience is well known. Nevertheless, from one point of view it is surprising. According to him, it is the same human reason which (designated as practical reason) can consider one and the same thing as being good and (designated as conscience) bad at the same time. To understand his conception of the conscience, it is necessary to look into the historical background. St. Thomas developed his conception of conscience explicitly in relationship with the Augustinian tradition of higher and lower reason, with Aristotle's explication of an incontinent man and with the contemporary discussion on synderesis; and he connected it implicitly with the problem of 'two wills' from the Augustinian tradition and with the question of man's possibilities to act rightly. The article aims to explain the content of Aquinas' conception of the conscience on the basis of its historical background.