I’ve been using this phrase—argument by definition—for some time, but I’m not sure where I got it from. In addition, I wasn’t sure until recently what exactly I thought I meant by it. I could intuitively identify such arguments, but as I lectured on the idea last week I had to sit down and wrote out precisely what I’m trying to identify when using the phrase.
Here’s a good empirical argument:
X is defined as anything with characteristics Y, and upon empirical investigation, some/many/most/all X have Z.
Here’s an argument that looks like an empirical argument but is not.
X is defined as anything with characteristics Y, and upon empirical investigation, all X have Y.
This is passing off an a priori judgment as if it were an a posteriori judgment. “Hey, I did some research, and it turns out that all bachelors are unmarried!” No kidding.
I see something like this from time to time in arguments that all religions are identical. It doesn’t take the form of the argument above, but it is similar. This one goes something like this:
Claim: All religions are intrinsically peaceful.
Objection: Medieval Christianity was not apparently so.
Counter-Claim: Medieval Christianity was not authentically religious.
In the end, what this amounts to is putting “peaceful” in the definition of “religion,” such that anything not apparently peaceful is not a religion. So the claim looks like an empirical one—all religions are peaceful—but it’s actually an argument by definition.