Q. E. D.

“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?

Photo: “Degrees of Craft” by Bryan Atchison, taken at CUA’s School of Architecture and used by permission.

4 Comments Add yours

  1. Cara Dimarcantonio says:

    Understanding why something is true can make a fact more meaningful to an individual. Knowing something is true does little to affect change or enrichment in the individual, but understanding its truth can. The understanding in itself is another truth. For example, to say something happened is only partially the truth. To explain why that occurred would be to give the whole truth.

  2. Madeline Aghajanian says:

    Even though something may appear obviously true, you need to understand why it is true to fully interpret the situation. For example, if we think about the discussion on 9/11, everyone knows what occurred that day but you need to know why it happened in order to fully understand the situation. The whole truth is given by explaining what occurred.

  3. Jaesen Evangelista says:

    Geometry was actually one of my favorite subjects in high school. I was more of a spacial thinker than an algebraic thinker–anything algebra killed me. And the reason why I think geometry is such a fascinating math is that it is entirely based on proofs. It requires people to prove what they’re saying and not just claim that it is “obviously true.”

    I’m sure everyone is familiar with the O.J. Simpson case or at least heard of the case before. And if not that, perhaps you’re familiar with Law and Order. Either way, all of these things are based on proofs. When someone is accused of a crime, there needs to be proof before punishment can be made. That is where the idea “innocent until proven guilty” comes from. You cannot accuse someone of something because it is “obvious” that they committed the crime. Everyone must be given a chance to prove themselves just because you never know when your judgment can be wrong and have dire consequences. Everything must be backed up by solid support, whether we are talking about medical conditions, politics, or heck, even triangles. We all need proof.

  4. Diego Amaya says:

    The value of understanding something is true is that you know why it is true not just because it is obvious. You understand it better and when someone asks you why it it’s true, you will be able to explain it to them and not just say because it is.

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