Philo Judaeus De Posteritate Caini 168

[168] but it is not correct to say that the living God is visible, that is rather an abuse of language, arising from referring God himself to his separate acts of power; for even in the passage cited above, he does not say, "Behold me," for it is wholly impossible that God according to his essence should be perceived or beheld by any creature, but he says, "Behold! it is I," that is to say, behold my existence; for it is sufficient for the reasoning powers of man to advance so far as to learn that there is and actually exists the great cause of all things, and to attempt to proceed further, so as to pursue investigations into the essence or distinctive qualities of God, is an absolute piece of folly;