Dialogue of Justin Martyr with Trypho

"Does philosophy, then, make happiness?" said he, interrupting.

"Assuredly,' I said, 'and it alone.' "

"'What, then, is philosophy?' he says; 'and what is happiness? Pray tell me, unless something hinders you from saying.'"

"'Philosophy, then,' said I, 'is the knowledge of that which really exists, and a clear perception of the truth; and happiness is the reward of such knowledge and wisdom.'"

"'But what do you call God?' said he. "

"'That which always maintains the same nature, and in the same manner, and is the cause of all other things -that, indeed, is God.'"