The essential task of the investigation of self-deception is nothing more than establishing the boundary of it, herein known as the distinction problem of self-deception. Such a boundary is necessary for distinguishing the phenomenon of self-deception from other similar phenomena, especially wishful thinking, and sheds light on the future research of other theoretical questions posed by the phenomenon. Although philosophers have reached a vague consensus on certain necessary elements involved in the phenomenon of self-deception, there is no general agreement on (...) their details, leading to a lack of canonical literature regarding the question of what makes self-deception a unique phenomenon. In this thesis, I begin by attempting to re-establish the initial definition of self-deception by illustrating the vague consensus in the current discussion of self-deception. Then, several representative views are examined to uncover the reason for their failure to capture the distinction between self-deception and other kinds of irrationality. These findings are then used to clarify what the basic structure of self-deception should be. Finally, I develop an account revealing that the distinction problem is related to the study of the necessary attitude involved in self-deception. My analysis shows that a special kind of instability inherent in the necessary attitude involved in self-deception is the key to establishing self-deception as a distinctive case of motivated irrationality. (shrink)