This vain presumption, of understanding everything, can have no other basis than never understanding anything. For anyone who had experienced just once the understanding of one single thing, thus truly tasting how knowledge is accomplished, would then recognize that of the infinity of other truths, he understands nothing.
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Ch. 15, p. 354; note: though this statement is incorporated into the story as one Galileo spoke, it is actually a quotation of one he historically made in his Dialogue Concerning The Two Chief World Systems as translated by Stillman Drake.