In Parmenides 133a8-134b5, Plato discusses the ‘greatest difficulty’ for the, or a, theory of forms. One of its conclusions (and the one I focus on) is that we can’t know forms. Elsewhere, Plato offers an epistemological argument for the existence of forms: we can have knowledge only if we know forms; knowledge is possible; hence forms can be known. GD threatens this argument. How should we understand the ‘greatest difficulty’ argument? How should we, and how might Plato, respond to it?