 Externalism in the philosophy of mind contends that the meaning or content of a  thought[1]  is partly determined by the environment. The view has garnered attention since it denies the traditional assumption, associated with Descartes, that thought content is fixed independently of the external world. Apparently under this assumption, Descartes also believed that he could know the content of his thoughts while suspending all judgment about his environs. (Indeed, such knowledge was thought indubitable.) Yet if externalism is correct, this may well be a mistake. As we shall see, externalism can suggest that Descartes is unable to know that his own thought represents, say, elm trees (vs. beech trees) without knowing that it is elms (and not beeches) that the thought is connected to in the world. But if such worldly knowledge is a prerequisite, then Descartes could not know the content of this thought just “from the armchair,” so to speak. So there seems to be a conflict between externalism and such armchair knowledge of one’s own thought contents (for short: “armchair self-knowledge”). The question whether this conflict is real is what drives the contemporary debate on externalism and self-knowledge.