In the second of his Meditations, Descartes examines a piece of wax. “Where does my knowledge of the wax come from?” he asks.

I should forthwith be disposed to conclude that the wax is known by the act of sight, and not by the intuition of the mind alone, were it not for the analogous instance of human beings passing on in the street below, as observed from a window. In this case I do not fail to say that I see the men themselves, just as I say that I see the wax; and yet what do I see from the window beyond hats and cloaks that might cover artificial machines, whose motions might be determined by springs? But I judge that there are human beings from these appearances, and thus I comprehend, by the faculty of judgment alone which is in the mind, what I believed I saw with my eyes.

(Meditation II)

Although Descartes’ point is a sceptical one, it raises an interesting point. On what basis do we claim knowledge of the internal experiences of other people? On the one hand, our experience of ourselves is the most certain thing – as Descartes himself would argree. However, if we cannot formulate a clear argument to go beyond this view, we are left with what is called solipsism, or the idea that we can only really have knowledge about our own mental states.

Descartes himself tried to base his knowledge of the outside world on the Cogito – his certainty of his own existence – and the fact that more reliable knowledge seems to be clear and distinct. However, as we saw in our discussion of the Meditations, both the notion of clear and distinct ideas and the cogito itself were problematic.