The law of identity
The law of identity states that whatever is, is, and whatever is not, is not. [Put some other ways, A=A. Or If A is true, then A is true.] It is often overlooked because it is a mere tautology. It’s true by definition. But there actually is a practical application to it. Suppose you’ve got two kids, and you come home one day and find that there’s a bowling ball that has smashed through the front of the TV. You know that one of the kids did it, but you’re not sure which one right away. Well the way you determine who did it is by applying the law of identity. The person who did it is the person who did it. So what you do is you take your two kids, sit them down on the couch, and you determine if the person who smashed the TV is identical to one of the persons sitting on the couch. Once you determined that whatever is true of one of the persons sitting on the couch is also true of the person who smashed the TV, then you know you’ve got the right person because they are identical. They are the same person. If you can discover something true of the person who smashed the TV that is not true of one of the persons sitting on the couch, then you can eliminate that person as a suspect. The law of identity is how we know that Steve is our philosophy instructor and not somebody else. If whatever is true of our philosophy instructor is also true of Steve, then Steve is our philosophy instructor. You can know that I am not the philosophy instructor because there is at least one thing that is true of the philosophy instructor that is not true of me.
[After Dale's question on yesterday's blog, I got to thinking that the law of identity is really used even more mundanely than the example above. We use the law of identity constantly. If you go in WalMart, you use the law of identity to find your car when you come back out. You use the law of identity to recognize people you know. You use the law of identity to make sense out of what people say. For example, when somebody says, "My dog barks," they don't mean, "My dog does not bark." They mean "My dog barks."]