What should I ask Niall Ferguson?
Fichte is known as a key interpreter of Immanuel Kant who removed the idea of the "thing-in-itself," i.e. reality apart from how we interpret it, from Kant's system. Kant had described how human faculties impose structural features like space, time, number, and causality on the world of our experience, i.e. the phenomenal world, but still retained as essential to ethics and religion the idea that there is a true world behind the phenomenal that we can't know, but we can act in accordance with. Fichte thought this latter idea wasn't necessary, and the part of this popularization of his system (the more difficult work being his Wissenschaftslehre, a.k.a. The Science of Knowledge) that we read for this discussion ends in pure idealism, which means that when you know anything, you're really just being aware of a part of yourself. The work is a series of introspections, like Descartes' Meditations, but doesn't ask "what can I know?" so much as "what is my vocation?" i.e. "what am I supposed to do?" ...which in turn requires that he interrogate what he can know.