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Achilles: I must confess, Mr. T, you got me with that Zeno nonsense. But, trick me no more you will! I have decided to build a clock. A most perfect clock that will measure the exact moment that I pass you in our race.

Tortoise: A great idea, Achilles, but how will you know your clock has the correct time?

Achilles: Simple, I will compare it against the best clock available.

Tortoise: But, how do you that clock is correct?

Achilles: Hmm, I hadn’t thought about that.

Tortoise: I doubt that you can build a clock that will ever tell the right time, because I don’t think you can even describe what a correctly functioning clock looks like.

Achilles: Of course I can! Everybody knows how to tell if a clock is correct or not.

Tortoise: OK, then. Why don’t you give me a specification for your clock and I will build it for you.

Achilles: OK. The clock must have a round face with 60 evenly distributed marks around the edge. It must have the numerals 1-12 imprinted around the circumference evenly spaced every five tick marks with 12 at the top. It must have a big hand and a little hand. The big hand points to the hour and the little hand points to the minute. The hands must point to the correct time at all times.

Tortoise left and returned a while later with clock in hand. Achilles examined it.

Achilles: The hands are pointing to exactly 12PM, but it is now 4PM. Furthermore, there is nothing inside this clock. Its hands don’t move! This doesn’t even come close to meeting the requirements of my specification. Mr. T, I am disappointed. I thought you would do much better.

Tortoise: Not so fast, my friend. I claim my clock is more accurate than any clock you could dream up based on the specification you gave me.

Achilles: I don’t need to dream too hard to think up a clock that actually has hands that, you know, move.

Tortoise: OK, suppose I built a clock that had springs, gears, and an escapement, you know, all the things that clocks have. Let’s say that my clock is accurate to one minute a day. Furthermore, let’s say I set it to the exact correct time based on some reference clock that we agree upon.

Achilles: Now you are talking.

Tortoise: But, my dear Achilles, that clock would be far less accurate than the clock I just built you!

Achilles: How is that possible?

Tortoise: You are right about the clock I built. It doesn’t move. But, it is exactly right twice every day, no more, no less.

Achilles: I see that, but twice per day is nothing to write home about.

Tortoise:Now, let’s analyze your clock. It is accurate at the time I set it, but it immediately becomes inaccurate because it loses one minute per day. It will only be accurate every 720 days. So you see, my clock is actually far more accurate.

Achilles: Ha, Ha. Very funny. You know that all you have to do is make my clock more accurate.

Tortoise: OK, let’s say it only loses one second per day instead of one minute. Then, it will only be accurate one every 43,200 days. I would rather have the clock that is one minute slow.

Achilles: OK, then make it more accurate.

Tortoise: But, this is not possible. No matter how accurate you make it, it is not possible to be exactly synchronous with the reference. You are doomed to failure. And, paradoxically, the closer you get, the less accurate your clock is!

Achilles: I think you are trying to get Zeno back into this conversation.

Tortoise: No, I am just trying to point out that your clock specification is very poor.

Achilles: OK, then how do we fix it?

Tortoise: How do you think we should fix it?

Achilles: We amend it to say that the clock can be off by plus or minus one minute with respect to the correct time.

Tortoise: But my dear Achilles, this just delays the inevitable. Your clock will be correct for the first day by that measure, but will not be accurate again for another 718 days. My clock will be accurate for four minutes every day. My clock is still twice as accurate as yours.

Achilles: Mr. T, you are missing the point. Yes, my clock may become inaccurate after a day, but the user can always reset it to the correct time.

Tortoise: Ah, but that was not part of the specification. Do you want to amend your specification yet again?

Achilles: Yes, I will amend it to say that the user can reset the watch whenever she/he wants.

Tortoise: OK, I will produce a new clock that meets your specification.

Tortoise left and, after a time, came back with a new clock.

Achilles: this clock is no better than your previous clock, Mr. T. It still doesn’t move and there is nothing inside it to make it move.

Tortoise: But, it meets your specification. My first clock had the hands glued to the time 12 O’clock. They could not be moved. This clock has hands that can be moved and therefore, be reset at any time. And, it still has the advantage of being more accurate than your clock when the user decides to not bother with resetting it. On top of that, if the user is not happy with the time displayed by my clock, they can simply set it to to any time they desire.

Achilles: You are trying to use technicalities to avoid admitting you are wrong. You know as well as I do what a clock should look like and how it should behave. The specification I gave you is good enough.

Tortoise: OK, I’ll give you one last chance. If it is that simple, you should be able to give me a specification that gives me no choice but to give you what you want. If I don’t have to think too hard about how to beat you, your specification can’t have been that good and you must concede the point. Do you agree?

Achilles: Alright, I’ll give it one last shot. I am going to add one more condition to my specification. Assume we have a reference clock. It doesn’t matter how accurate it is, as long as its accuracy is acceptable to me. And I am not going to specify that because it doesn’t matter. What I am going to specify is that the clock that you give me must have its hands move at the same rate as the reference clock plus or minus 1/60 of the reference clock’s rate. That is, if we assume that the reference clock’s big hand turns exactly 360 degrees in one hour, your clock’s big hand can be off by one minute per hour. Similarly, the hour hand rate must match the reference clock’s big hand rate with the same accuracy.

Tortoise: Whew, that was quite a mouthful!

Achilles: I will concede that specifying things completely is harder than I thought, but I think this is finally bulletproof. You will have to build me a clock with hands that move, so I am not willing to concede yet the main point that I cannot guarantee that I get the clock I want.

At that, Tortoise left yet again, presently to return bearing two clocks.

Tortoise: First, I will tell you that you used one of the most common copouts when it comes to specification. You specified how it should work rather than its behaivor. The function of a clock is to tell time, not have hands that go round in circles. That is implementation details. However, having said that, here is my clock.

Achilles: It is the same clock! The hands still don’t move. You violated my specification!

Tortoise: No, I didn’t. You specified the hands should move at the same rate as a reference clock. Here is my reference clock. It is an atomic clock with a digital display. It has no hands. Therefore, I am not violating your spec. Yet, atomic clocks are the most accurate in the world, so I think even you would concede that it is accurate enough to meet your specification.

Achilles: OK, I give up. I concede. There appears to be no way to completely specify even something as simple as a clock in a bulletproof way.

Tortoise: You are finally seeing the light, my friend. I suggest you stick to things you are good at, like foot races with slow moving creatures like me.

[with apologies to Lewis Carroll and Douglas Hofstader]


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  1. […] in Uncategorized In previous posts, I have tried to illustrate that writing complete, unambiguous specifications is hard, if not […]

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