Warning! This is posted more in a spirit of getting it off me chest than actually informing any of my regular readers of anything they didn't already know, or of being entertaining…
Here's one I seem to have run across, from theists, too many times of late:
I define "God" as an uncaused entity who necessarily exists. Therefore God necessarily exists, by definition.
Now, I don't know where this little gem comes from (though it has a certain whiff of William Lane Craig about it, if you want to know where I'd lay my money), but I would like to point out, in the politest terms possible that it's… well it's bullshit. Sorry! Far from proving anything, either way, about the existence, or not, of gods, it doesn't even make rhetorical sense.
Firstly, and most obviously, it's more circular than a very circular thing from Circleland. I mean, a person could get dizzy! "He exists because I define him as having to exist and I know I'm right because here's the definition…"
Secondly, we don't derive the properties of something ("uncaused," "necessarily existent") from its dictionary definition, no matter who writes the dictionary. We derive the definition from a description of the observed properties of the object in question. As far as gods go, in other words, you need to show that they exist, that they're uncaused and that they're necessarily existent, before you write those properties into your definition.
Fourthly, I don't see what it's even supposed to prove, from the point of any monotheist. If a god is something which has, of necessity, to exist, then Zeus has to exist; Pan has to exist; every god who's ever been worshipped has to exist. Which, I'd think, would be just a leetle bit of a problem for anyone claiming that their god is the only god…
Fifthly, and finally, consider the context. The definition is dragged out, ninety-nine times out of a hundred, when the origin of the universe is being discussed. Which is fair enough, since we're talking of a creator-god, but hold on… The theist will have already dismissed the big bang theory on account of the problem of "everything needing a cause," so they bung that "uncaused" term into the definition to "explain" why their god needs no cause. So, hold on, we can have an uncaused god but not an uncaused big bang? Why? Don't get me wrong; I can see a problem with infinite progression whichever idea we go with unless we throw something uncaused into the mix, if we want to explore the idea of the very first cause, but on what grounds do you claim that one can be caused but the other cannot? Whim? Personal preference? Toss a coin? Nope, if "uncaused" is possible, then you have to admit that any "first cause" or "initial condition" may be uncaused.
Sorry, it's a bad argument, from the point of view of logic, philosophy or rhetoric. And it doesn't even come close to being science.
So, please, stop using it. It's not big, and it's not clever.
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