The basic idea here is that the kinds of things that actually exist determine the kinds of
ideas we can have; it’s impossible to think of a thing that has absolutely nothing to do with reality. The Meditator isn’t denying that we can imagine fictional things—instead, he’s saying that reality provides the
building blocks for our ideas, and we cannot have an idea with more parts than we have building blocks. Even the wildest fantasies are just new combinations of things that already exist: for instance, one-legged tie-dye unicorns don’t exist, but the idea of them is made up of ideas about elements that
do exist, like legs, colors, horns, and so on. In contrast, it is truly impossible to imagine a color that does not exist (and is not just a combination of other colors that
do). This is why the meditator concludes that the cause(s) of an idea must be more
perfect than the idea itself. In this context,
perfection means that it belongs to a deeper level in the taxonomy that Descartes has laid out: an infinite substance is more perfect than a finite substance, which is more perfect than a specific object.