Rationalism
Experience informs, but cannot ground necessity: seeking in the principles the senses alone cannot guarantee.
1637 – 1716
A continental current born in the heart of the 17th century, in the age of the scientific revolution of Galileo and Kepler, when the success of mathematics in studying nature seemed to call for a rebuilding of knowledge on equally secure foundations. Its lineage runs from , who opens the way with the and the Discourse on the Method, to , who draws from it in the a geometric system, and on to , who crowns it in the Monadology. The problem it addresses is that of foundations: on what can certain knowledge be secured after the collapse of scholastic physics and under the threat of ? Against the of and , which derives all knowledge from experience, the school would be held to be superseded by 's synthesis, which keeps its a priori component while stripping away its claim to know things in themselves. Even so it fixed the modern ideal of science for centuries and remains one of the two poles around which classical epistemology is organised.
The senses yield only the particular and the contingent: the alone, with its innate ideas, reaches necessary and universal truths.
Certain knowledge is built by starting from self-evident first principles and deducing the rest, more geometrico, as one proves a theorem.
does not have the last word: there are truths no doubt can shake, and reason attains them.
Nothing exists or happens without a reason: the real is intelligible through and through, and the of each thing can be found.
Things have intelligible essences that the mind defines a priori: one can deduce their properties without awaiting anything from experience.
Justification
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 with its . turns the 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 sees in the senses only impressions, sets this confused knowledge of the first kind apart from knowledge through , the only kind that grasps things through their causes.
Nothing is in the intellect that was not first in the senses, except the intellect itself.New Essays on Human Understanding, II, 1
Descartes, Meditations on First Philosophy, III · Leibniz, New Essays on Human Understanding, II, 1
Justification
To answer it is not enough to pile up probable opinions: a truth is needed that no argument can shake. makes this his strategy, pushing as far as the hypothesis of the evil demon so as to keep only what survives it: the very act of thinking, which outlasts every possible doubt. Once that rock is found, the task is to propagate its certainty to the rest of knowledge, and the model for this is geometry: ordering knowledge more geometrico, from self-evident axioms to their necessary consequences, as deduces the in the manner of the geometers. The certainty aimed at is not a psychological assurance but the compelling self-evidence of demonstration.
I think, therefore I am.Discourse on Method, IV
Descartes, Meditations on First Philosophy, II · Spinoza, Ethics, opening of Part I
Justification
Every justification, on pain of an infinite regress, must come to rest on principles that depend on no other: rationalism settles the matter by holding that these stopping points are first truths grasped with self-evidence by intellectual intuition, not ever-revisable sense data. This is a thoroughgoing foundationalism: knowledge is built like a deductive edifice, from axioms toward theorems, each step preserving the certainty of the last. Against , which backs justification onto singular impressions unable to ground the necessary, rational self-evidence alone can serve as the base, because it shows itself indubitable in the very act of being conceived.
Descartes, Meditations; Spinoza, Ethics
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Justification
Truth is conceived as the adequacy of an idea to its object, but an adequacy secured from within by rational coherence: for Spinoza the order and connection of ideas is the same as the order and connection of things, and the true is its own standard. The Cartesian criterion of the clear and distinct idea and the deductive chaining of reasons draw the school toward an ideal in which correspondence and coherence converge.
Spinoza, Ethics, II, proposition 7 and scholium to proposition 43 · Descartes, Meditations on First Philosophy, IV
Justification
Mathematics is the model of all certain knowledge: Descartes, inventor of analytic geometry, makes the order of mathematical reasons the paradigm of method, and Leibniz, co-inventor of the infinitesimal calculus, dreams of a universal characteristic that would turn every disagreement into a calculation. Confidence in a unified deductive science inclines the school toward a strong epistemic optimism.
Descartes, Discourse on the Method, part two · Leibniz, Discourse on Metaphysics
Justification
The school is firmly realist about knowledge: reason does not construct its object, it reaches the real order of things as it is in itself. The order and connection of ideas is the same as the order and connection of things, writes Spinoza, the true idea agreeing with what it represents. Descartes' clear and distinct ideas grasp the real essence of things, and the intelligible order of Leibniz's monads is the very architecture of reality: to know is to read the structure of the world, not to fabricate it.
The order and connection of ideas is the same as the order and connection of things.Ethics, II, proposition 7
Spinoza, Ethics, II, proposition 7 · Descartes, Meditations on First Philosophy, V · Leibniz, Monadology, §§ 56-62
Justification
The norm of assent is evidence: one should affirm only what the understanding perceives clearly and distinctly. Descartes makes error a fault of the will judging beyond what the understanding clearly conceives, and prescribes withholding assent outside of evidence. For Spinoza it is the that carries a necessary and true affirmation. Assent is thus regulated by the clarity of the idea, not by a will to believe.
Descartes, Meditations on First Philosophy, IV · Spinoza, Ethics, II, proposition 49
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Justification
The rationalists converge on an intellectual highest good: the perfection and joy of the knowing soul. Descartes' firm contentment, Spinoza's blessedness as the intellectual love of God, and Leibniz's increasing perfection place the good life in the accomplished exercise of reason, where flourishing and virtue unite, more than in pleasure.
Spinoza, Ethics, V · Descartes, The Passions of the Soul · Leibniz, Monadology
Justification
The self is thought as a substance or a simple thing, not as a bundle of impressions: Descartes defines the subject as a thinking thing (res cogitans), a substance whose whole essence is to think, and Leibniz makes the soul a monad, a simple indivisible substance. The substantialist position is clearest in these two thinkers.
Descartes, Meditations on First Philosophy, II · Leibniz, Monadology, §§ 1-7
Justification
Deduction needs premises: to demonstrate a thing's properties one must start from a definition of its essence, and that essence must be graspable by the understanding alone, without recourse to experience. Hence the school's essentialism: things have intelligible, definable essences that reason reaches a priori. thus deduces all the properties of substance from its definition alone, and grasps the essence of matter as pure extension, through the analysis of the piece of wax rather than through the senses. This is the exact reverse of , for which no essence precedes the observation of particular cases.
Spinoza, Ethics, I, definitions and first propositions · Descartes, Principles of Philosophy, part I
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Justification
The rationalists converge on an intellectualized virtue ethics: the good lies in the perfection and power of the rational soul. Descartes' , Spinoza's active blessedness, Leibniz's wisdom make moral life the flourishing of a character enlightened by reason, more than obedience to a law or the calculation of effects.
Spinoza, Ethics, IV-V · Descartes, The Passions of the Soul
Justification
A corollary of the metaphysical realism: truths, practical ones included, are objective and accessible to reason, not relative to a culture or an individual. Spinoza deduces the true good from the nature of substance, more geometrico, as a necessary and universal truth, and Leibniz grounds justice on eternal truths inscribed in the divine understanding. Reason, everywhere the same, gives access to a universal normative order.
Spinoza, Ethics, IV, preface and propositions 18-37 · Leibniz, Meditation on the Common Concept of Justice
Justification
God is not received from faith but required by reason: without him the deduction would not hold. needs him to move beyond the cogito and guarantee that our clear ideas are not the work of a deceiver, and he proves him in the Third and Fifth . identifies him with the single, necessary substance from which everything follows (), and deduces him from the as that which accounts for the existence of the world and grounds the pre-established harmony. The doctrine is not uniform: Descartes' personal creator God, Spinoza's immanent God and Leibniz's perfectly rational God differ deeply. What they share is a family resemblance: a divine principle that reason establishes and that secures the intelligibility of the world, rather than a primarily personal, revealed God.
Descartes, Meditations on First Philosophy, V · Spinoza, Ethics, I, propositions 11 and 14 · Leibniz, Monadology, §§ 36-45
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Justification
If reason can know the real a priori, it is because the real is rational through and through: nothing exists or happens without a reason that makes it intelligible, on pain of leaving brute facts opaque to thought. This postulate, which raises into the , governs the whole of rationalism and divides it from the of , for whom causality is only a habit of associating impressions, not a necessary connection. Physical nature is read mechanistically, through figures and motions in and through necessary laws in , but finality returns at the metaphysical level, with subordinating the mechanism of bodies to ends willed by God.
Leibniz, Monadology, §§ 31-32 · Descartes, Principles of Philosophy, part II
Justification
The passions stem from confused ideas and must be mastered by the understanding: Descartes offers a mechanical analysis and an art of governing them through knowledge, and Spinoza holds that an emotion ceases to be a passion as soon as we form a clear and distinct idea of it. Correcting affect by reason draws the school toward a moral intellectualism.
Descartes, The Passions of the Soul · Spinoza, Ethics, V, proposition 3