But, one certainly shouldn't have a blanket positive attitude either.
For example, I can't do open-heart surgery, and I'm pretty sure I'll never be able to, either.
- I would have to need a certain amount of manual dexterity, first. I might have that, and I might not.
- I'd have to be able to operate without blanching, or worse, at the sight of blood. I might be able to, but, to be honest, I'm not at all sure about that.
- I'd have to be able to operate without fear of screwing up. And, given my temperament, and upbringing, I'm not at all sure about passing that hurdle.
- And, I'd have to graduate medical school to legally do open-heart surgery, as well as to learn about how to do it of course.
Because, to graduate medical school, I'd need:
- Some pre-med classes
- Money to take some pre-med classes
- Med school
- Money for med school.
That said, physical or mental limitations due to heredity, or due to poor educational background, are also externalities. If my hands aren't steady enough to operate, it's not my fault. Or, if I'm not a "sales-assertive" enough person, it's not (largely) my fault, even as positivity gurus still try to tell us "we're all in sales," even though I know that's not true.
And, per that, if I "can't" do something because, per Polonius in Hamlet I must "To (mine) own self be true," that's certainly not a fault or a limitation. After all, per Daniel Goldhagen in "Hitler's Willing Executioners," way too many Nazi-era Germans found that they "could," indeed, murder Jews for no other reason than that they were Jews.
But, back to the more everyday "can't."
Even if one is wrong, or overly self-pessimistic for thinking and believing this, others of us should ask why this person is saying that. Beyond the fear of failure, maybe it's a fear of lack of education or training. And, in the "we're all in sales" modern American business world, that latter concern is nothing to laugh at indeed. When many companies (especially in a business field like mine) are stretched ever thinner, and there's little to no training or support offered for new job demands, or, in other, Fortune 500 type cases, companies have the money they could offer training, continuing education, etc., but in a Social Darwinian way, instead expect you to shell out on your own, then it's a legitimate issue.
Beyond that "I can" vs. "I can't" is too black-and-white. Theoretically, Michael Jordan could hit a AA-level curve ball, but far less easily than he could dunk.
So, don't be too doubting of yourself. But, when you are reasonably doubting of either yourself, or the larger situation around you, don't be too hard, either. The world needs pessimists in general; a few more of them on Wall Street might have prevented the financial meltdown.