17 But, they say, if you are not satisfied with our opinion, do you point out, tell us yourselves, what is the Deity's form. If you wish to hear the truth, either the Deity has no form; or if He is embodied in one, we indeed know not what it is. Moreover, we think it no disgrace to be ignorant of that which we never saw; nor are we therefore prevented from disproving the opinions of others, because on this we have no opinion of our own to bring forward. For as, if the earth be said to be of glass, silver, iron, or gathered together and made from brittle clay, we cannot hesitate to maintain that this is untrue, although we do not know of what it is made; so, when the form of God is discussed, we show that it is not what you maintain, even if we are still less able to explain what it is.
Source: Against the Heathen (New Advent)