Eusebius tells us that it was the opening sentence of Protagoras' treatise on the Gods, and it is attested by numerous citations. It runs as follows: "About the gods, I am not able to know whether they exist or do not exist, nor what they are like in form; for the factors preventing knowledge are many: the obscurity of the subject, and the shortness of human life." The last words have sometimes been omitted, but they are important. They indicate the ground upon which Protagoras took up his position and the nature of his agnosticism. All that mattered to him was what could be known; and the Greek word (eidenai) that is used twice in this sentence means, precisely, knowledge: not belief, not faith.