Meditation 12.28

 To those who ask, Where hast thou seen the gods, or how dost thou comprehend that they exist and so worshippest them, I answer, in the first place, they may be seen even with the eyes; in the second place, neither have I seen even my own soul, and yet I honor it. Thus then with respect to the gods, from what I constantly experience of their power, from this I comprehend that they exist, and I venerate them.
 

(podcast episode) (original Greek)