How does Plato explain that a particular object is beautiful or just?

In Platonism, Plato explains that a particular object is beautiful or just through his theory of forms. He posits that entities such as "the Beautiful" and "the Just" exist as abstract, eternal, and unchanging realities that are independent of both mental activity and the sensible world.1234

A particular object—such as a beautiful vase or a just act—is considered beautiful or just because it "participates in" (a term translated from the Greek metechei) the corresponding Form. While sensible particulars are imperfect, fleeting, and "roll around between being and not-being," the Forms themselves are the fundamental constituents of reality that provide intelligibility to the sensible world.124

Because Forms are not located in space or time, they are inaccessible to sense perception and can be known only by the philosophically schooled intellect. Thus, for Plato, a particular thing possesses a quality only insofar as it stands in this relationship of participation with the perfect, abstract Form of that quality.341