Meditations, Book 12, Section 21

Marcus Aurelius — The Emperor

When people ask you, "Where have you seen the gods? How do you know for certain that gods exist, since you worship them so devotedly?" I answer this way. First, they are visible to the eye in some way. Second, I have never seen my own soul, yet I respect and honor it. So with the gods — through daily experience of their power and care for me and others, I know certainly that they exist. That is why I worship them.

Marcus Aurelius — The Emperor Original

To them that ask thee, Where hast thou seen the Gods, or how knowest thou certainly that there be Gods, that thou art so devout in their worship? I answer first of all, that even to the very eye, they are in some manner visible and apparent. Secondly, neither have I ever seen mine own soul, and yet I respect and honour it. So then for the Gods, by the daily experience that I have of their power and providence towards myself and others, I know certainly that they are, and therefore worship them.