Apparent inability to draw certain types of object

I am largely bemused by people claiming that they can't draw a specific sort of thing, yet they can draw other things. They are usually referring to things that they have in front of them, so it's not failure to remember what it look like. Hands are a good example, bicycles another. Surely, if you can see it, you can draw it? Some light was shed on one aspect of this recenty when a colleague was talking about her friend, who could paint skies and water, but not detail; and she hadn't got the hang of perspective, either. So, I get the difference between the broad sweeps of skies and water, and the fine work of detail. That's technique, and precision. But, while formal perspective is useful for understanding, it isn't strictly required in order to draw, or paint, what you see. Oh, dear. Now I'm wondering about pre-renaissance lack-of-perspective. I suppoe there were more conventions to adhere to then. And no photographs to ensure that you get it right. There isn't anything that I would say that I couldn't draw. Some things are trickier, and some take a longer while, but if it can be seen, it can be drawn, given a source that doesn't vanish. What do you think? Do you have to know what something is, and how to draw it, before you can capture its image on paper? Do you have things you "can't" draw?