I'm about to complete my B.A. in philosophy, and I want to pursue a PhD in philosophy. I'm aware that it's a long, difficult, lonely process and that doesn't scare me off. But I can't help compare myself to the grad students who TA in my school's philosophy department. In particular, I lack the clarity of argument (especially verbal) and originality of thought that they regularly exhibit. I know these skills are developed, but I wonder if any part of them is innate. That is to say, it seems to me unlikely that everyone who desires to get a PhD and go into teaching is capable of doing so, just like not everyone's capable of being a professional basketball player, regardless of desire or training. Obviously you should go for your dreams, but it's important to be pragmatic and recognize your limitations.
My question is this: what would it look like if someone wasn't cut out for a PhD program, and how would they know? I ask because I want to be sure (as much as is reasonable) that I'm capable of being accepted to and eventually completing a program before I invest a lot of time and money into the application process.