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.'