When people say “I can’t” I’ll sometimes encourage them to say “I decided not to” or something instead. Nobody can predict the future, so maybe nobody can know for sure whether somebody would be able to do something if they tried some more times. However, a person has a right to decide to stop. They may judge that it’s so unlikely they would succeed that it’s not worth trying; and doing it may not be worth a tremendous amount to them. I also have a right to my opinion that maybe they can.
realsocialskills answered:
You have a right to your opinion, but you don’t have the right to have them respect your assessment of their abilities. You especially do not have the right to have them take your opinion into consideration when they’re deciding what they can and can’t do.
Inability to do things is real. And yes, I may sometimes be wrong about my inability to do things, but taking it seriously when I think I can’t do something matters. Even if I’m wrong.
There’s a difference between deciding I don’t want to do something, and deciding that I think I am incapable of something, or that doing the thing is unacceptably risky for me.
Even if other people think I’m wrong - I still have the right to assess what my limits are and act accordingly. And even though I will sometimes mistakenly think that I am unable to do something I am actually capable of, “I can’t” is still a vital part of my vocabulary.
There’s a difference between not wanting to do a thing, and reaching the conclusion that I’m probably not capable of doing the thing and that trying is hurting me.
I need to be able to acknowledge that I have limits in order to manage them correctly, and do what I can instead of pretending that enough willpower makes everything possible.
So does everyone else. In particular, people with disabilities who have been taught that we’re not allowed to take physical limitation seriously. But being disabled and physically limited isn’t a moral failing. It’s just a fact of life that sometimes needs to be accounted for.

nonary said:

Am I really unable to do [activity x], or am I just letting myself off the hook by saying I can’t? I struggle with this question pretty much every single day. My physical disability doesn’t restrict my movement, so I can do a lot of the things able-bodied people can. I can walk. I can run. I can hold a pencil. I can type. I can even stand up in a moving bus. 

But should I do these things?

It takes me a lot of energy to do them. I can walk pretty far without getting tired, but if I run I’m out of breath in about ten seconds. I need to factor in recovery time and energy levels, and sometimes it’s not worth it to push myself, even though I like to walk. Writing by hand is extremely labor-intensive, so I can’t take notes in class and still pay attention to what’s being said. Typing is easier, but I still type slowly. I don’t touch-type, though. Maybe I could learn, but it’d take a very long time to build up the muscles I’d need, and it probably wouldn’t improve my typing speed very much. Should I learn to touch-type? No, I shouldn’t. If it wouldn’t improve efficiency, shouldn’t I instead pursue another solution?

It’s easier to just say that I can’t touch-type. It’s easier just to say that I can’t walk twelve kilometers. “I can’t” doesn’t necessarily mean “I decided not to.” Sometimes it means “I won’t because it’s a terrible waste of my time and energy.”  And sometimes it’s the best answer.

realsocialskills said:

Yes, exactly.

And that’s not a special disability-specific use of “I can’t”, either. That’s a standard meaning for people *without* disabilities use - it’s just more socially accepted when the thing you can’t reasonably do is something most people can’t reasonably do.