The concept of religion necessarily splits into two categories, the substantive principles that are polarized about the idea of the revered object, and the manner of' applying them in the private behavior of the worshiper or in the public institutions of the state. Theory and practice are not conflicting terms. Philosophy, however, has its roots in principles and hesitates to shape the external forms in which its counsels may be expressed. Therefore the studies here are confined to the didactic issues at stake, and religion is not defined except as the thinker defines it himself.
The philosophers appearing in this book are: Plato, Aristotle, Epicurus, Marcus Aurelius, Augustine, Leibniz, Spinoza, David Hume, Immanuel Kant, Auguste Comte—ten thinkers of the western world, from many ages and diverse stocks, who offer us their beliefs and conclusions. Though many distinguished names could be added to this list, the philosophies here expounded will serve to reflect the unflagging interest of humanity in this subject.