This paper develops a new argument against free will, understood as the principle of alternate possibilities (PAP). This principle has been central in debates around free will and moral responsibility; however, it is almost always stated in terms of bodily, rather than mental action, and it is therefore mainly understood as the possibility to physically act differently, rather than to think differently. This chapter offers an argument against the PAP, understood in this latter way, which is termed the possibility of alternative thought (PAT). It argues on psychological grounds that it is impossible for a subject to think differently than it does in a given situation. First, I make the possibility of alternative thought explicit, explain what it means for a self to entertain a thought (or conscious content), and I define a notion of conscious control that is required for the argument. I then offer a taxonomy of content types, based on the control structure that governs their genesis. In the third section, I analyze the scenarios of content generation for each type of content, to determine whether they can satisfy the PAT. It will be shown that this cannot be the case.