Descartes’ first sceptical argument is the argument from error. In the First Meditation of his Meditations on First Philosophy, he writes:

“Whatever I have up till now accepted as most true I have acquired either from the senses or through the senses. But from time to time I have found that the senses deceive, and it is prudent never to trust completely those who have deceived us even once.”

The argument rests on the principle that what has once deceived us cannot be completely trustworthy. Our senses, argues Descartes, have deceived us in the past. Square towers appear round when distant; straight sticks may appear bent when in water; coloured objects appear in shades of grey in darkness.

If our senses have proved fallible in the past, though, then why think that we can trust them now? If we want to avoid error, then we must be cautious, and so ought to withhold our assent from beliefs based on perception. This, though, means that we must doubt much of what we usually take ourselves to know.

Descartes entertains this argument only briefly. Though our senses may sometimes err, he suggests, they only do so in unusual circumstances. What we clearly and distinctly perceive, he suggests, we can trust is so. He therefore sets aside the argument from error, and moves on to the argument from dreaming.