The Causal Role of the Form of Beauty

The primary object of knowledge, if you like, would be the form of beauty. But for whatever reason, plato seemed to think that his forms were causes. And then you could maybe use that to check whether your opinions about things being beautiful were, in fact true or not. It just seems clear from the dialeges that he didn't, right? So something can serve both functions, or at least it's coherent or plausible for plato to have thought that something can service both functions or fill both roles.