As we now know where the idea of externality comes from, it seems that we're in a position to decide if this idea corresponds to real objects existing outside of us. The idea of externality is given to us in the idea of space. So the question might be asked as follows: Does the idea of space correspond to an objective reality? But we're not quite ready to answer this question, which amounts to asking if objects really exist in space. This can't be answered until we've decided that objects really exist, and this is what we're going to consider in this lecture. The question of the objectivity of the idea of space is actually part of another, more complicated question of knowing if the laws of the mind are also the laws of things, to which we won't turn for a while.

So in order to know if anything really exists outside of ourselves, we'll have to proceed in some other way. Using an inductive approach, let's imagine that we have a sensation and then try to determine its cause. If we decide that the cause lies within us, we'll conclude that there's no such thing as the nonself. But if we decide that the cause lies outside of us, we'll conclude that the external world does exist.

How do we determine the cause of a phenomenon? Logic can be our guide.