Kant and Hegel 1780s–1830s, the deep root
Everything downstream of Kant is a fight that starts here, well before the twentieth-century rooms that later redistributed it. Immanuel Kant's Critique of Pure Reason (1781) is built around a claim that looked, to his predecessors, like a contradiction in terms: the synthetic a priori. To see why, Kant crosses two distinctions. A priori judgments are known independently of any particular sensory episode — they claim strict universality and necessity (it must be so in every possible case). A posteriori judgments arise from experience — "the sun is warm," this crow is black — and so give particular instances, not that things must be that way everywhere. Kant's own clarification matters: all knowledge begins with experience in a causal sense (you would never have arithmetic without having learned to count), but a judgment can still be a priori if experience cannot justify it. Once you have the concepts, "7 + 5 = 12" is not confirmed by looking harder at objects; nothing you see could overturn it the way a warm morning could overturn "the sun is always cold."
Cross that with a second cut. Analytic judgments are explicative: the predicate is already contained in the subject-concept — "all bodies are extended," or "all bachelors are unmarried." Analyzing the concept clarifies it; it adds no new information. Synthetic judgments are ampliative: the predicate is not packed into the subject — "all bodies have weight," or "all the crows I have seen are black." That crossing yields four boxes, two of which Kant treats as empty or uninteresting. Analytic a posteriori drops out: if the predicate is already in the concept, you do not need experience to extract it. Analytic a priori is real but thin — pure clarification of concepts. Synthetic a posteriori is ordinary empirical knowledge: weight added to body, blackness added to crow. The load-bearing claim of the first Critique is the remaining box. There are synthetic a priori judgments — known through pure reason, independent of justifying experience, yet genuinely ampliative. Empiricists after Hume had treated ampliative knowledge as always contingent on the senses; rationalists after Leibniz had treated necessary, universal knowledge as always analytic. Kant's wager is that mathematics, the basic laws of natural science, and the legitimate core of metaphysics sit in that third kind. "7 + 5 = 12" is not analytic: you can turn over the concepts of seven, five, and addition as long as you like without finding twelve already inside them; the sum has to be constructed. "A triangle has three sides" is analytic a priori; "the interior angles of a triangle sum to 180°" is, for Kant, synthetic a priori — universal and necessary, yet not readable off the mere definition of triangle; you need construction (Euclid's style of proof, applied to the essential properties of any triangle, not to this crooked chalk drawing). "Every event has a cause" is the same kind of claim for physics. Without judgments like these, Kant argues, you would have either empty logic or a heap of contingent observations, and no bridge to objective knowledge of a shared world.
How such judgments are possible is what transcendental idealism is for. Human experience, on Kant's account, does not mirror reality "as it is in itself"; it is structured in advance by innate frameworks of the mind. The forms of intuition — space and time — are a priori structures of sensibility, not properties we read off things; they are what make geometry and arithmetic possible as synthetic a priori sciences in the first place. The categories of the understanding — causality, substance, and the rest of Kant's twelve — are the concepts through which the mind synthesizes the manifold of sensory data into coherent, law-governed experience, which is why something like "every event has a cause" can be known as necessary for any possible experience of objects, not merely induced from habit as Hume had concluded. The cost of that solution is the famous split this whole lineage keeps returning to: phenomena (things as those forms and categories necessarily structure them) versus noumena (things as they are in themselves, permanently unreachable by that structuring).
Buried in the same book is the quieter move that ties the desire genealogy to the sibling neuroscience essay's Descartes. The "I think" that must be able to accompany all of a person's representations — the transcendental unity of apperception — is a formal precondition for having unified experience at all, not a persisting soul-substance. Descartes's Meditations had treated the cogito as revealing a thinking thing (res cogitans), a mind-substance really distinct from body; the Paralogisms chapter of the first Critique argues that inference is illegitimate. From "I think" you get a thinking act that can accompany representations; you do not get metaphysical knowledge of a simple, immortal, substantial soul. Rational psychology, on Kant's diagnosis, confuses a formal condition of experience with an object known in itself. That is the bridge this whole document keeps crossing under different names: a subject as something a system does, not something a system has — the same crack Lacan and Hofstadter reopen on the grounding page, and the same Cartesian knowing subject Heidegger will later displace with Dasein, below.
A Scottish side-branch answers the same early-modern corridor from the other direction. Thomas Reid's Inquiry into the Human Mind on the Principles of Common Sense (1764) refuses the way of ideas that runs from Descartes through Locke's Essay into Berkeley and Hume: the claim that what we immediately know are mental representations, with the external world (or other minds) always an inference away. That setup, Reid argues, makes radical skepticism look mandatory once you take it seriously — so he rejects the setup. Common sense, for him, names first principles of belief (that there is an external world, that memory is generally trustworthy, that other minds exist) that philosophy has no standing to dissolve without using them. Hume is the skeptic who woke Kant; Reid is the other exit from the same corridor — restore trust in perception and judgment rather than rebuild objectivity through transcendental forms. The genealogy follows Kant into Hegel and desire, not Reid into Scottish common-sense realism; the pin is here so the epistemology fork is marked. Locke the political theorist of property and personhood sits on the history page beside Hobbes and Mills.
Hegel's Phenomenology of Spirit (1807) treats Kant's phenomena/noumena wall as an artifact of not yet being dialectical enough, and replaces the static split with a developmental one: consciousness passes through and overcomes its own successive, self-undermining shapes — sense-certainty, perception, understanding, self-consciousness, reason — on the way to a fuller self-knowledge. Its most consequential single passage for everything that follows is lordship and bondage (the master-slave dialectic): self-consciousness only becomes genuinely self-conscious through a life-and-death struggle for recognition with another self-consciousness, producing an unequal master/slave relation that then dialectically inverts, since the slave's formative labor on the object gives the slave a more genuine self-relation than the master ever gets from simply being obeyed. This is the actual text Kojève will read, a century later, four chapters into a much longer book, as fundamentally a theory of desire.
A separate, stranger Hegel passage — the 1805–06 Jena manuscripts' "night of the world," consciousness as an abyss where disconnected images surface and vanish — becomes, over a century later, Žižek's source for the self-relating negativity covered in the neuroscience piece's Žižek section, and Adrian Johnston's "transcendental materialism" there borrows its name directly from the Kantian sense above: conditions of possibility, not something beyond nature.