This paper explores the question of the unity of Transcendental Idealism at the end of Eighteenth Century German philosophy, given that it circulated in different versions, Kant’s Critique [of humans’ rational powers] and Fichte’ System of Science [Wissenschaftslehre]. Both thinkers take the transcendental turn. They base conceptual investigations not on facts or empirical evidence, but on the possibility of a situation; they are idealists since they look inward to the spontaneity of the agent/knower for explanation, not the environment, stimulus, or sensory given. Reason can fathom only what it has constructed.