For the study of the great writers and thinkers of the past, historical imagination is the first necessity. Without mentally referring to the environment in which they lived, we cannot hope to penetrate below the inessential and temporary to the absolute and permanent value of their thought. Theory, no less than action, is subject to these necessities; the form in which men cast their speculations, no less then the ways in which they behave, is the result of the habits of thought and action which they find around them.