On Friedrich Wilhelm Joseph Schelling’s System of Transcendental Idealism (1800), Parts 1 and 2. This continues from Ep. 273 and features Mark, Wes, and Seth.
If you're an idealist, and so think that all existence is somehow in our minds, then the key to any knowledge whatsoever would have to be an understanding of the mind. Schelling thought in particular that in the act of recognizing that you are a self, then you get the subjective (the self part) and the objective (the "other" that the self is encountering) all at once. An animal is aware of objects, sure, but its awareness is not "consciousness" in the sense of implicitly involving itself as a subject. So even in (human) perception itself we've got (for Schelling) that joint introduction of subjective and objective, and in the act of turning inward to make explicit that act of self-consciousness that's already contained in regular perception, we can really uncover that fundamental relationship between consciousness and world.
It's the whole task of Schelling's book to pull apart the "moments" within self-consciousness; this ends up being the whole point of philosophy. This isn't quite as weird as it sounds, as even for Descartes, an analysis of what self-consciousness amounts to is the beginning of philosophy, providing justification for all other knowledge. But this is not just a matter of establishing a foundation for warranted belief, but an inroad for actually gathering knowledge. Part of that introspection that is getting at the self is also getting at the contents of all of our perceptions, everything that everyone tells us, and everything that we do by instinct or custom: That's what it means to say that self-consciousness includes revelation of the objective.
However, just saying that what we learn in any arena is going to have something to do with the contents of our consciousness would not be sufficient to justify the claim that the moment of self-consciousness in particular is the linchpin of all knowledge. There has to be something more that looks like the foundationalism of Descartes. But it can't be a matter of establishing through self-consciousness an indubitably true proposition that you can derive the rest of knowledge from, because the experience of self-consciousness is not a proposition, and in fact is required according to Schelling for us to make sense of propositions and truth at all. Rather, self-consciousness for Schelling is the very act of self-consciousness itself; it's an act that creates its own object, and thereby (allegedly) creates objectivity and hence the possibility of truth (i.e. correspondence of object and a thought we have representing that object).
Schelling's text makes an argument similar to the cosmological argument for the existence of God (or even more accurately, Avicenna's). Objects in our experience must depend on (be "conditioned by") something, which in turn must depend on something else until we hit some self-supporting entity (the "unconditioned"). Self-consciousness, as an act that creates its object, can serve as that ultimate ground for knowledge. It provides its own ground. God or atoms cannot for for Schelling's ultimate stopping point, because they're still part of the objective world that his project aims to justify knowledge of.
A central but controversial part of Schelling's picture is that logical truths (which seem like purely elements of thought, i.e. on the subjective side) don't make sense unless we also have an idea of objects (the objective side). You might think that a statement like "A=A" is a truth of pure reason: anything will be identical to itself. But "anything" includes "thing" (or "entity" would also be enough). In some sense, it would still be true that A=A even if the universe never existed at all. But for us to actually understand that claim to be true, we have to have in mind that there really are objects, and so that the universal logical truth applies to objects like that. The logical/subjective is not prior to the empirical/objective; rather, we need them both in order to think.