The second sceptical argument put forward by Descartes is the argument from dreaming:

“How often, asleep at night, am I convinced of just such familiar events—that I am here in my dressing-gown, sitting by the fire—when in fact I am lying undressed in bed! … As I think about this more carefully, I see plainly that there are never any sure signs by means of which being awake can be distinguished from being asleep. The result is that I begin to feel dazed, and this very feeling only reinforces the notion that I may be asleep.”

It is impossible, Descartes argues here, to distinguish waking experiences from those in dreams. Dreams can be vivid and convincing. Often, we do not realise that the experiences that we took to be real were dreamed until we wake up. Whatever experiences we are now having, then, might turn out to be dreams; we could wake up at any second.

This argument, unlike the argument from error, threatens to shake our confidence in our senses on a deep level. Whereas the argument from error suggested that our senses might sometimes deceive us, that some of our beliefs based on perception might turn out to be false, the argument from dreaming suggests that our senses might systematically deceive us, that all of our beliefs based on perception might be dreamed and turn out to be false. The scepticism engendered by the argument from dreaming is thus much more thoroughgoing than that engendered by the argument from error.

There are still, however, limits to the argument from dreaming, as Descartes immediately identifies:

“Suppose then that I am dreaming, and that these particulars—that my eyes are open, that I am moving my head and stretching out my hands—are not true. Perhaps, indeed, I do not even have such hands or such a body at all. Nonetheless, it must surely be admitted that the visions which come in sleep are like paintings, which must have been fashioned in the likeness of things that are real, and hence that at least these general kinds of things—eyes, head, hands and the body as a whole—are things which are not imaginary but are real and exist.”

Although dreams may mislead us in some ways, convincing us that we are doing or seeing things that we actually are not, they cannot mislead us in all ways. Dreams, though they may be fictions, must borrow from the real world. Though it may be that we are only dreaming that we have eyes, heads, and hands, and so that we do not know this about ourselves, there must be some real eyes, heads, and hands on which our dreams are based. Our dreams may deceive us, but not completely.

Descartes continues to propose a slightly more modest, but similar defence:

“… although these general kinds of things—eyes, head, hands and so on—could be imaginary, it must at least be admitted that certain other even simpler and more universal things are real… This class appears to include corporeal nature in general, and its extension; the shape of extended things; the quantity, or size and number of these things; the place in which they may exist, the time through which they may endure, and so on.”

On consideration, Descartes decides that it may be that our dreams of our bodies are not based on real eyes, heads, hands, and so on. Those dreams must nevertheless, he insists, contain some elements that are real: substance, shape, number, and time, for example. The argument from dreaming, though it may cause us to have extensive doubts about the reliability of perception and about our view of the world, should not lead us to doubt the world itself. In this, Descartes proposes an early version of Ryle’s counterfeit coinage argument.

There is another limit to the argument from dreaming: there are certain truths, it seems, that are learned by reason rather than through the senses. Whatever problems the argument from dreaming may cause for our trust in sense-perception, it does not touch our trust in reason. As Descartes notes, “whether I am awake or asleep, two and three added together are five, and a square has no more than four sides.”

Descartes next proceeds to an argument more extensive in scope even than the argument from dreaming: the argument from deception.