In case anyone ever asks you

Some months back, I got into an argument with a creationist who insisted with an air of triumph that “Survival of the fittest is a tautology”—i.e., meaningless.
How do we know they’re the fittest? Because they survive! Why do they survive? Because they’re fit! The terms are never defined, therefore natural selection is based on circular logic, so forget the evidence, natural selection is a myth and evolution is WRONG!
Now I know from experience most creationists aren’t going to be deterred by counter-arguments, but I think it’s useful for evolutionary believers to have an argument handy just in case. So here it is.
Survival of the fittest is not a scientific term, it’s simply a shorthand. And Darwin did not base his theories on such a fuzzy concept (he didn’t use the phrase—borrowed from his cousin Herbert Spencer—until well after he’d written Origin of Species).
Darwin’s theory was that if organism A has an inherited trait that enables it to produce more offspring than organism B, the next generation will include more As than Bs. Then those As will breed more, and the generation after that will have an even greater proportion of As. And natural selection, if the situation continues, will produce an ever-growing number of As overtime while B fades away (it’s slightly more complicated in practice, but not too much).
This isn’t a tautology. And simple observation has confirmed the pattern time and time again. Natural selection works (even a lot of creationists admit this, they just don’t believe anything actually evolves as a result).
So if the subject comes up ….