We are accustomed to think and to speak of the physical universe as having objective existence, externally to ourselves; and as being in objective existence quite apart from any question of our perception of it. Even the idealistic philosopher regards the universe so, except when he is reflecting mentally upon other possibilities. The realistic view is the primary view. Idealism is an afterthought. And the idealist, if he is an investigator in physical science, cannot avoid acting upon the postulate of realism. Nothing else gives him a foothold for work. He has to regard the universe “as if” it were real. All our philosophies are necessarily based upon postulate; all our science is founded upon faith.