Philosophers of all ages have wondered how an agent can fail to set himself up to undertake an action that he knows is in accordance with what would be best to do. This is a core issue. How can we fail to do what we know would be best to do, if it is characteristic of human agents to act on account of reasons? What exactly is the relationship between reasons and action? And what kind of knowledge does the agent really have in the first place if he is unable to bring himself to act on it? It should come as no surprise that Thomas Aquinas too was intrigued by these issues. Under this background, I would like to reexamine Aquinas’s account of this phenomenon of acting contrary to our best knowledge. And I think that a close look at his reading of Aristotle can still provide us with some new and important insights. Contemporary philosophers use the expression ‘weakness of will’ when they discuss the inability to act on one’s knowledge, yet Thomas Aquinas speaks primarily about ‘incontinentia’, which is the common Latin translation of Aristotle’s ‘akrasia.’ Within this view, I reveal that ‘incontinence’ which Thomas Aquinas is discussing forms certain special circumstances in the connection with Aristotle. With this in mind, I conduct my research in the following way. First, I talk about incontinence and different ways of knowledge. Second, I look into incontinence and sensual desire in syllogism. Third, I examine closely about the link between incontinence and will. Forth, I examine passions as a prime mover of incontinence. Finally, I draw the some positive conclusions regarding the philosophical lessons one can draw from Thomas Aquinas’s understanding of incontinence.