Then you're stuck in a futile search for some assurance that the things you think are real aren't delusions, fantasies, or elaborate hoaxes. Ayn Rand also defended a unique theory of concepts , or abstract ideas. For her, the ability to form concepts was the primary function of the faculty of reason , the possession of which is the essential difference that sets humans apart from all the other animals. Nominalism says that no two things are the same, and that we know what each other means because there are social conventions in place that govern how we do things with words. The notion that there could be literally abstract ideas is supposed to be incoherent.
Classification is entirely pragmatic.
It undercuts nominalism at its root by exposing the subjectivism behind it, while it rejects classical realism by showing the pointlessness of the intrinsicist view of essences. With the power of reason fully vindicated, there is no role for faith in living a good life.