why do cars have brakes?

I don't remember where I was when Kevlin Henney asked me:

Why do cars have brakes?

but I do remember his answer as a definite light-bulb moment. What's your answer?

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A very common answer is

to stop

or some variation thereof. In other words, to brake. It reminds me of the noun-verb thing again. Lights light. Irons iron. Compilers compile. Brakes brake. But that was not Kevlin's answer. Kevlin's answer was:

So you can drive faster.

His answer feels paradoxical which usually means there's a deep truth. Brakes allow you to stop when you want. Brakes allow you to stop where you want. Software tests are like brakes. They help you drive faster.

I'm also reminded of the P.W.Bridgman quote at start of chapter 6 of Jerry Weinberg's Introduction to General Systems Thinking:

...it is better to analyze in terms of doings or happenings than in terms of objects or static abstractions.

Thinking about brakes allowing you to stop is thinking statically. Thinking about braking allowing you to drive faster is thinking dynamically.

If you liked this post you might like the follow up Why do car drivers brake?.