“Q. E. D.” is an abbreviation for the Latin phrase “quod erat demonstrandum.” In English, it means “which was to be demonstrated.” It’s an abbreviation that sometimes occurs at the end of logical or geometrical proofs. As the title of today’s reading (CUA Primer, pages, 36–37), it refers to a geometrical proof, in the first part of the reading, by the Greek mathematician Euclid. There he explains how we know that the length of one side of a triangle is shorter than the lengths of that same triangle’s two other sides put together. In the second part of the reading, a later thinker, the philosopher Proclus, argues against people whom he says are making fun of the proof by saying that the conclusion is obvious. Even if something appears obviously true, what value might there still be in understanding why it is true?