On knowledge: is there any circumstance in which I am without doubt free from the sceptical possibility that all things are in my imagination only?
There is yet another way to doubt what Descartes supposed we could know with certainty. It seems that one might also wonder if we might be fallible even in the ways in which we categorize our own experiences. Descartes imagines a very powerful malin genie (an evil demon) who would delight in confusing us about anything and everything. Couldn't such a being even confuse us about how to categorize experiences as thoughts (rather than, say, headaches or tickles). To identify something as a thought (and not a headache or a tickle), we have to be able to rely on the conceptual apparatus by which we make such distinctions. It is not obvious that our conceptual apparatus on this or any other subject is invulnerable to error or confusion--especially given the best efforts of some evil divinity!