This last conclusion bears on the next discussion with which I will terminate this lecture.

This question can be formulated thus, Can alternative temporal series be found in nature?
A few years ago such a suggestion would have been put aside as being fantastically impossible.

It would have had no bearing on the science then current, and was akin to no ideas which had ever entered into the dreams of philosophy.

The eighteenth and nineteenth centuries accepted as their natural philosophy a certain circle of concepts which were as rigid and definite as those of the philosophy of the middle ages, and were accepted with as little critical research.

I will call this natural philosophy 'materialism.' Not only were men of science materialists, but also adherents of all schools of philosophy.