 Here, Descartes considers three kinds of idea: innate ideas, adventitious ideas, and what are sometimes called factitious ideas. The categories are determined by considering the possible origins of the ideational contents presented or exhibited to the mind. The first category includes ideas whose contents have their origin in his nature (qua thinking thing). An example is his idea of what thought or thinking is. The third category includes ideas whose contents have their origin in the contents of other ideas. An example would be the idea of Pegasus, where its content is drawn perhaps from the contents of the idea of a horse and the idea of a bird. Adventitious ideas, however, appear at least at first glance to be importantly different, since Nature has always taught him, he says, to think that they are “derived from things existing outside me” (AT VII 38; CSM II 26). The category arises in part from ordinary (pre-philosophical) experience: “…I know by experience that these ideas do not depend on my will, and hence that they do not depend simply on me. Frequently I notice them even when I do not want to: now, for example, I feel the heat whether I want to or not, and this is why I think that this sensation or idea of heat comes to me from something other than myself, namely the heat of the fire by which I am sitting” (AT VII 38; CSM II 26). An account of their origin, he suggests, may require an appeal to things that exist external to, or independently of, his mind. Adventitious ideas include sensory ideas; ideas that originate in sensory experience—such as the ideas of the Sun or the Moon, but also the more simple ideas of colors, sounds, heat, cold, and the like.