Immanuel Kant
A priori knowledge, in Western philosophy since the time of Immanuel Kant, knowledge that is acquired independently of any particular experience, as opposed to a posteriori knowledge, which is derived from experience. …
All these developments led directly to the philosophy of Immanuel Kant, the greatest philosopher of the modern period, whose works mark the true culmination of the philosophy of the Enlightenment. Historically speaking, Kant’s great contribution was to elucidate both the sensory and the a priori elements in knowledge and thus to bridge the gap between the extreme rationalism of Leibniz and the extreme empiricism of Hume.
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Kant conceived of reason as being at the very heart of the philosophical enterprise. Philosophy’s sole task, in his view, is to determine what reason can and cannot do. Philosophy, he said, “is the science of the relation of all knowledge to the essential ends of human reason”; its true aim is both constructive (“to outline the system of all knowledge arising from pure reason”) and critical (“to expose the illusions of a reason that forgets its limits”).
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Kant is known for his theory that there is a single moral obligation, which he called the “Categorical Imperative“, and is derived from the concept of duty.