Rationalism

The starting point is a paradox of learning set out in the Meno: one can search neither for what one already knows nor for what one is wholly ignorant of, being then unable to recognise it. Recollection cuts the dilemma: having contemplated the Forms before birth, the soul discovers nothing absolutely new but remembers, as the slave of the Meno recovers a theorem under the mere guidance of questions. Platonism is thus a consistent rationalism (the doctrine that makes reason, not the senses, the source of true knowledge): since sensation bears only on the changing and yields mere opinion, true knowledge cannot come from the senses but from reason turning back upon itself. This is the exact reverse of empiricism, for which nothing is in the intellect that was not first in the senses.

Plato, Meno, 80d-86c, recollection · Plato, Republic, 509d-511e, the divided line

The decisive point is that a necessary and universal truth cannot come from the senses: these yield only particular, contingent cases, never the « it cannot be otherwise » of mathematics. A non-sensory source is therefore required, and it is the understanding with its innate ideas. Leibniz turns the empiricist maxim against itself: if nothing is in the mind that was not first in the senses, one must still except the intellect itself, whose structures make any experience thinkable. Where Hume sees in the senses only impressions, Spinoza sets this confused knowledge of the first kind apart from knowledge through adequate ideas, the only kind that grasps things through their causes.

Descartes, Meditations on First Philosophy, III · Leibniz, New Essays on Human Understanding, II, 1

Since the senses deceive us and yield only confused qualities, they cannot ground a certain science: only the understanding, which conceives geometrical extension clearly beneath the sensory appearances (the analysis of the piece of wax), reaches the nature of things. Descartes infers that the founding concepts, the idea of God, of extension, the mathematical truths, cannot come from experience: they are innate ideas placed in the understanding. All science then reduces to the intuition of simple natures and the deduction that links them, on the model of mathematics, which sharply separates him from the coming empiricism, for which nothing is in the mind that was not first in the senses.

Meditations on First Philosophy, III and V · Rules for the Direction of the Mind, III

The doctrine answers the paradox of the Meno: one can seek neither what one already knows nor what one wholly ignores, having no way to recognize it. Plato concludes that all learning must be a recollection, anamnesis, of Forms the soul contemplated before birth. The proof is experimental, not asserted: questioned by the method of questions alone, the slave boy of the Meno, who never learned geometry, recovers a theorem on his own, which rules out his having received it through the senses. Knowledge therefore comes from within, from reason turned toward the intelligible, against all empiricism.

Meno, 81a-86c · Phaedo, 72e-77a

Against the empiricism of Locke, for whom nothing is in the understanding that was not first in the senses, Leibniz replies: nothing, except the understanding itself. The mind is not a blank slate but carries within it innate ideas in potency, like veins in marble; necessary truths (logic, mathematics, metaphysics) cannot come from experience, which yields only particular cases. This is an avowed rationalism, tempered by acknowledging the role of the senses for truths of fact.

New Essays on Human Understanding, preface and book II, ch. 1 · New Essays, II, 1, §2 (the veins in the marble)

True knowledge proceeds from adequate ideas by deductive chaining, not from sense experience, the source of confused ideas (the first kind of knowledge). Its summit is the third kind of knowledge, intuitive science, which grasps things in their necessity from the essence of God: an unabashed rationalism.

Ethics, II, prop. 40, scholium 2 (the three kinds of knowledge) · Treatise on the Emendation of the Intellect

On brahman, which is neither perceived nor inferred from the world, the only valid source is revelation (śruti), the word of the Upaniṣads: sense experience gives only the manifold, and reasoning alone cannot establish the absolute. But Śaṅkara is no fideist: reason (yukti) is indispensable to interpret Scripture, resolve its apparent contradictions and refute rival schools, and the teaching saves only when it becomes direct experience of the self. The primary material is thus not empirical but scriptural and rational, which places him on the side of reason without making him a pure rationalist, the authority of revealed testimony remaining first.

Commentary on the Brahma Sūtras, I.1.3-4 · Upadeśasāhasrī