It's through external perception that the external world becomes known to us. But does this world really exist? This is the question we'll address in the next few lectures. It can be subdivided into two other questions:

1. Does anything exist outside the self?

2. If something does exist outside the self, is it as we perceive it?

Before answering these questions, there's another we have to answer first: Where do we get the idea of externality, or – as it's also called – of the nonself?

An idea can have only two kinds of origins. Either it's somehow given to the mind or it's constructed by it, a work of the mind resulting from some kind of intellectual labor.

Let's see if the idea of externality is constructed.

A number of philosophers – from different philosophical schools – have responded that it is. Cousin was of this opinion, as was Stuart Mill, whose theory on the matter was the best developed. According to Mill, the idea of externality is constructed in the following way: All we know about the external world comes through sensation, which is – by its very nature – subjective. When as adults we have a sensation of color, of course, we immediately conclude that a colored object exists. But how do we reach this conclusion? This is what must be explained. By itself, a sensation is purely affective and subjective. So how could it give us the idea of externality?