[ruby-talk:5404] Object.foo, setters and so on

OK, here is what I think I know.

These three fragements differ only in that the last one defines a
private method, no?

class Object
  def foo
    ...
  end
end

def foo
  ...
end

def Object.foo
  ...
end

And isn't it true that a method like bar= must be called with a receiver?
Then why won't this work?

def bar=
  ...
end

self.bar = 5

It tells me that bar is private. Why? What is my fundamental misunderstanding?

Hal