If you want to learn to read Ancient Greek or play the guitar, you probably can. Not every thing is in reach for every person, but usually if someone wants to do something of this nature it is possible--at some realistic level of success. Realism itself, though, is kind of a trap, if confused with a pessimistic "know your limits" kind of thinking. Realism means knowing that it takes time and dedication to being good at something, but that time and dedication will produce tangible results. Realism is a certain clarity of thought about how much you really want a particular goal and how much serious time you can devote to it. It means removing purely mental obstacles to see what's actually preventing you from moving forward. For example, it is the thought that "Greek is hard" that might be holding you back, not the hardness of Greek itself. The actual difficulty of the task can be approached, but the idea of difficulty in the abstract is insurmountable.

They say it takes 10,000 hours to achieve mastery in any field of endeavor, but a lot of things are easier than that. Baking pretty decent bread does not take 10,000 hours of practice--or many other normal talents that a lot of people can develop. So the level of "realism" is going to vary as well. Someone who says they could never learn to bake bread is probably wrong. They don't really want to learn badly enough to make it worth their while.

Inherent talent only comes in tangentially to all of this. For example, someone might put in the hours and still not be great. But I can assure you the person will achieve a respectable degree of mediocrity-- which might be enough for their purposes.