It is therefore an urgent requisite for physical science and for philosophy to examine critically the grounds for this supposed necessity.

The only satisfactory method of scrutiny is to recur to the first principles of our knowledge of nature.

This is exactly what I am endeavouring to do in these lectures.

I ask what it is that we are aware of in our sense-perception of nature.

I then proceed to examine those factors in nature which lead us to conceive nature as occupying space and persisting through time.