None of Immanuel Kant’s writings can be understood without a clear recognition of the “Copernican revolution” in philosophy effected by his first critique, Kritik der reinen Vernunft (1781; The Critique of Pure Reason, 1838). Previously, the predominant rational tradition in Western philosophy was founded on the assumption of reason’s capacity for discovering the forms or essential structures characterizing all things. Whether the form of “treeness” was an innate aspect of every existent tree (as Greek philosopher Aristotle believed) or a transcendent form in which each existent tree participated (as Greek philosopher Plato held), the capacity of reason for perceiving such forms was not doubted. The medieval controversy over “universals” centered not in reason’s ability for such perception but in the nature of this rational activity.
From the first questioning of the nominalists, however, through the break between self and “exterior world” in the philosophy of René Descartes, doubt as to the precise authority of rational apprehension increased. Human error and empirical deception began to be seen as intervening between perceiver and perceived, thus raising powerfully the question of the criteria for truth. The Aristotelians, especially from the time of Saint Thomas Aquinas on, affirmed that knowledge begins with sense perception; however, because of reason’s capacity for extracting forms, human knowledge not only possessed the qualities of necessity and universality but also made possible an inductive knowledge of trans-empirical realities.
It was the empiricists, especially David Hume, who provided the most serious challenge to this rationalist claim. Centering his attack on the problem of universal causality (cause and effect as universally operative), Hume raised the question of necessity. On what grounds, he asked, can one insist that, of necessity, all “effects” have causes and, similarly, that such causes necessarily produce identical effects? Hume’s conclusion was that the category of causality, like all human ideas, is derived from sense impressions, having the status simply of a habitual assumption and expectation; human ideas are forever bereft of necessity.