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In the Meditations, after Descartes casts ideas as modes that represent objects to the mind, he divides ideas into kinds. He says: Among my ideas, some appear to be innate, some to be adventitious, and others to have been invented by me. My understanding of what a thing is, what truth is, and what thought is, seems to derive simply from my own nature. But my hearing a noise, as I do now, or seeing the sun, or feeling the fire, comes from things which are located outside me, or so I have hitherto judged. Lastly, sirens, hippogriffs and the like are my own invention. (AT VII 37–8; CSM II 26)
Here, Descartes considers three kinds of idea: innate ideas, adventitious ideas, and what are sometimes called factitious ideas. The categories are determined by what appears to him to be differences with respect to the origins of their contents. It will not be until later in the Third Meditation, and arguably not until the Sixth Meditation, that the three categories will be confirmed as genuine. The first category poses no difficulties, for he suggests that he can account for these ideas (their contents) — specifically the ideas of what a thing is, what thought is, and so on — by an appeal to his own nature. He is an existent thinking thing, and so the origin of the contents of the ideas he mentions, the objects they represent, can be traced to this fact about his nature. The last category of idea is also unproblematic, for he can easily account for them again by an appeal to himself. He puts them together, so to speak, out of other ideas that he already possesses. Adventitious ideas, however, do pose an immediate problem, 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). So, an account of their origin — that is, the origin of their content — may have to include an appeal to things that exist external to, or independently of, his mind. The problem is that at this stage in the Meditations certain forms of doubt that have yet to be resolved prohibit his adopting the view that there exist such things.
The belief that some of his ideas have their origin in things that exist external to, or independently of, his mind 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). Although Descartes begins the analysis by an initial examination of adventitious ideas, he ultimately extends it to cover the idea of God, which is the paradigm of an innate idea. For, as we learn just a few pages later in the Third Meditation, the idea of God is innate, and yet, as Descartes shows, it (or its content) must have its origin in God, the infinite substance, something that exists external to, or independently of, Descartes' finite mind.
This is not the only place where the distinction between the categories of innate and adventitious ideas is blurred. It arises again, for instance, in Comments on a Certain Broadsheet, published in 1648. There, Descartes casts innateness as a faculty (capacity) or tendency (AT VIIIB 358; CSM I 304), which aligns with what he had said in the Third Replies: “…when we say that an idea is innate in us, we do not mean that it is always there before us. This would mean that no idea was innate. We simply mean that we have within ourselves the faculty of summoning up the idea” (AT VII 189; CSM II 132). Descartes then turns, in Comments on a Certain Broadsheet, to applying this view to what in the Meditations were called adventitious ideas. Given that the human or embodied mind has the faculty or capacity to have sensory or adventitious ideas of pains, colors, sounds, and so on, occasioned on the occurrence or presence of certain motions in the brain, and nothing of the motions is transferred to the mind, and nothing resembling the pains, colors, and sounds are present in bodies, then the ideas of pains, colors, and sounds, he says, “must be all the more innate” (AT VIIIB 359; CSM I 304). Their possibility is in part rooted in an innate capacity of an embodied mind. And so, his adventitious ideas look to be innate in this sense.
One interpretation that resolves the above conflict casts innate ideas as ideas that underlie all other ideas, where the relationship between the innate ideas and all other ideas is understood in terms of the conditions of intelligibility (Nolan 1997, Lennon 2007, Nelson 2008). Consider, for example, the idea of the sun. Understood as a shaped thing, an analysis of this idea would reveal that the innate idea of extension (body) is in play, so to speak, for without it we simply could not conceive (or experience) the sun as shaped. Shape, recall, presupposes extension. As Descartes puts it in the Principles, everything “which can be attributed to body presupposes extension, and is merely a mode of an extended thing,” and so, “…shape is unintelligible except in an extended thing…” (AT VIIIA 25; CSM I 210). In this sense, insofar as a shaped thing is intelligible to us, which is to say that we have an idea of such a thing, the innate idea of extension is present. As some scholars have put it, the innate idea underlies the occurring idea of shape (Nolan 1997, Nelson 2008). This interpretation finds support in what Descartes says in a letter to Princess Elisabeth, dated 21 May 1643, where he introduces what he calls the “primitive notions.” These are what in other contexts he calls the innate ideas. He claims that they are “…the patterns on the basis of which we form all our other conceptions” (AT III 665; CSMK III 218). According to this reading, there is a sense in which innate ideas are always present, which puts pressure on the innateness equals capacity view, noted above. According to this reading, adventitious ideas rely on the innate ideas in the sense that the latter account for the intelligibility of the former. Consequently, the blurring between the categories of innate and adventitious ideas is resolved.
Despite the tensions that arise among the above-considered interpretations, scholars from both camps agree that with respect to innate ideas, Descartes recognizes at least three: the idea of God, the idea of (finite) mind, and the idea of (indefinite) body.