“In order to avoid simply describing experiments that have been done, we have to propose laws beyond their observed range. There is nothing wrong with that, despite the fact that it makes science uncertain. If you thought before that science was certain—well, that is just an error on your part.
To return then, to our list of conservation laws (fig. 14), we can add energy. It is conserved perfectly, as far as we know. It does not come in units. Now the question is, is it the source of a field?”