 In Republic V, Socrates returns to the issue of political rule by asking what change in actual cities would bring the ideal city closer to realization. The famous answer is that philosophers should rule as kings (473d). Trying to make the scandalous, even ridiculous, answer more palatable, Socrates immediately begins to explain what he means by philosophers. They are the ones who can distinguish between the many beautiful things and the one beautiful itself. The beautiful itself, the good itself, and the just itself are what he calls forms. The ability to understand such forms defines the philosopher (476a–c). Fully elaborated, this extraordinary theory holds that there is a set of unchanging and unambiguous entities, collectively referred to as being. These are known directly by reason in a way that is separate from the use of sensory perception. The objects of sensory perception are collectively referred to as becoming since they are changing and ambiguous (508d). This infallible grasp means that one knows what goodness is, what beauty is, and what justice is. Because only philosophers have this knowledge – an infallible grasp of goodness, beauty, and justice – they and only they are fit to be rulers in the city.