Kant calls these two sources sensibility and understanding. Sensibility is passive: it receives input from the world through the senses, delivering raw material in the forms of space and time. Understanding is active: it spontaneously generates concepts and rules through which that material is organised and interpreted. Neither operates on its own — the understanding without sensibility produces empty thought; the senses without the understanding produce a formless manifold that cannot be cognised at all.
This is Kant's famous formula: intuitions without concepts are blind; concepts without intuitions are empty. An intuition is a direct, immediate representation — a particular this, experienced here and now. A concept is a general rule — a way of classifying and unifying many particulars under a common heading. Knowledge requires both: particular data delivered by intuition, organised and made intelligible by concepts. The pure concepts of the understanding — the categories — provide the most fundamental rules for this organisation.
Kant's categories — unity, plurality, causality, substance, and others — are not derived from experience. They are the conditions that make any experience at all possible. Without the concept of causality, we could not experience one event as following from another; we would have only disconnected impressions. Hume had argued that we cannot find causality in experience and concluded it was merely a habit of mind. Kant accepts the first part but transforms the conclusion: causality is not in experience because it is presupposed by experience.
Kant positions himself against both the empiricists (Locke, Hume) who thought all knowledge derived from sensory experience, and the rationalists (Descartes, Leibniz) who thought pure reason could yield knowledge of the world through concepts alone. Both traditions were half-right: experience is necessary, but so is a priori structure. The Critique of Pure Reason attempts to explain precisely how these two elements cooperate to produce genuine knowledge, and to identify the disasters that result when reason tries to operate without any tether to intuition.
The two-stems doctrine is announced at the opening of the Transcendental Analytic, the second major section of the Critique. The pure concepts of the understanding — the categories — are derived and justified in the Transcendental Deduction, one of the most difficult passages in modern philosophy.