Is able to. The word that names a capacity rather than an act.
Old English cunnan (to know, to know how to do), from Proto-Germanic *kunnan, from a root meaning to know — the same root that gives 'know,' 'ken,' and 'cunning'; ability, at the root, is a kind of knowing.
Names a capacity rather than an act — what a thing is able to do, not what it is doing right now.