The way I like to put it is that instances and closures are duals: a closure can do exactly one thing, while an instance can, in general, do several things. That's why, when invoking an instance, you have to supply a method name that says which of the several things you want it to do, while when invoking a closure no method name is necessary.
Of course it's not a hard-and-fast distinction: you can have a closure that dispatches on one of its parameters to do one of several different things, and you can have an instance with only one method. But it's usually valid.
Of course it's not a hard-and-fast distinction: you can have a closure that dispatches on one of its parameters to do one of several different things, and you can have an instance with only one method. But it's usually valid.