SUMMARY There is a general agreement that Einstein's investigations have one fundamental merit irrespective of any criticisms which we may feel inclined to pass on them.

They have made us think.

But when we have admitted so far, we are most of us faced with a distressing perplexity.
What is it that we ought to think about?
The purport of my lecture this afternoon will be to meet this difficulty and, so far as I am able, to set in a clear light the changes in the background of our scientific thought which are necessitated by any acceptance, however qualified, of Einstein's main positions.

I remember that I am lecturing to the members of a chemical society who are not for the most part versed in advanced mathematics.

The first point that I would urge upon you is that what immediately concerns you is not so much the detailed deductions of the new theory as this general change in the background of scientific conceptions which will follow from its acceptance.