 One might object here that in order for humans to acquire knowledge of abstract objects in this way, they would first need to know that plenitudinous platonism is true. Linsky &amp; Zalta respond to this by arguing that plenitudinous platonism (or in their lingo, principled platonism) is knowable a priori because it is required for our understanding of any possible scientific theory: it alone is capable of accounting for the mathematics that could be used in empirical science no matter what the physical world was like. Balaguer's response, on the other hand, is based on the claim that to demand that platonists explain how humans could know that FBP is true is exactly analogous to demanding that external-world realists (i.e., those who believe that there is a real physical world, existing independently of us and our thinking) explain how human beings could know that there is an external world of a kind that gives rise to accurate sense perceptions. Thus, Balaguer argues that while there may be some sort of Cartesian-style skeptical argument against FBP here (analogous to skeptical arguments against external-world realism), the argument in (1)–(6) is supposed to be a different kind of argument, and in order to respond to that argument, FBP-ists do not have to explain how humans could know that FBP is true.