The Necessary Alarm Clock
The terms “necessary” and “sufficient” get thrown around a lot in the LSAT world these days. We at Manhattan LSAT use them to distinguish between two different kinds of assumption questions. They come up on the test in answer choices (for example, “The argument mistakes a necessary condition for a sufficient condition”). And usually by the fourth session of a course, students start making jokes like, “I had half a sandwich earlier, which was necessary, but not sufficient.” We all pretend not to love the joke.
But what do they mean? One of my favorite analogies for explaining the basic difference between necessary and sufficient is the alarm clock. We’ll call it the Necessary Alarm Clock.
Say I have a very hard time waking up. I’m a sad, awful person who hates morning, sunlight, and everything to do with happiness. I so dread being awake, in fact, that in order for me to get up in the morning, eight things need to happen:
1. My alarm clock goes off.
2. The smell of bacon drifts into my room.
3. I have dreamt of lilies and puppies.
4. Speaking of puppies, my dog is licking my face.
5. A marching band passes.
6. It’s not a Monday.
7. I am not hungover.
8. It is 72 degrees in my bedroom.
Again, in order to wake up, every single one of these things must occur.
So what is sufficient for me to wake up? 1-4? 5-8? 1, only? None of these. The occurrence of 1-8 is sufficient for me to wake up, and 1-8 only. Could we say, then, that 1-8 is also necessary for me to wake up? Sure! Each of these has to happen; that means they are all necessary.
How about my alarm clock going off? Is that sufficient for me to drag my caboose outta bed? No. 2-8 still has to occur. But is it necessary? To answer that question, we ask what would happen if it DIDN’T go off. If it didn’t go off, I’m still snoozing. So, yes, it’s necessary.
Finally, suppose 1-8 occurred, plus there were fourteen cheerleaders practicing in my kitchen. Would I wake up? Of course I would. The sum total of 1-8 plus the cheerleaders is sufficient to get me out of bed. But is the sum total of 1-8 plus the cheerleaders necessary to get me out of bed? No. The cheerleaders can be disposed of.
Yes, I wrote this post just to be able to close with that sentence.