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Rationalism is the doctrine that posits discursive reasons as the only possible source of all knowledge of the world. In other words, reality would only be knowable by virtue of an explanation by the causes which determine it and not by any revelation or intuition. Thus, rationalism means any doctrine that attributes to reason alone the ability to know and establish the truth.

In its classical sense, it is to postulate that reasoning consists in determining that certain effects result from certain causes, solely from logical principles; to the way in which mathematical theorems result from the hypotheses admitted at the start. Moreover, and in particular, the logical principles themselves used in reasoning have been known by deduction.

Rationalism is opposed to irrationalism and, historically, to empiricism.

Terminological clarifications

We commonly and identically find the expressions “modern rationalism” or “classical rationalism” to designate rationalism as it is formulated from Descartes to Leibniz, corresponding more or less to what we can call since Kant “dogmatic rationalism”.

  • Rationalism is dogmatic when reason, considered as the only determining source of knowledge, and by its only a priori principles, claims to reach the truth, particularly in the metaphysical field.
  • The phrase “modern rationalism” is intended to situate it in the history of thought in accordance with the usual terminology (the modern period beginning in the sixteenth century, after the period of the Middle Ages) and distinguishing it from the status of reason in ancient philosophy, as found in Plato and Aristotle for example.
  • The expression “classical rationalism” is intended to distinguish it from an expanded and renewed rationalism, “modernized”, by Kantian criticism and the contribution of the experimental sciences: “critical rationalism” for Kant and Karl Popper, “applied rationalism” for Gaston Bachelar.
  • We also find the expression “continental rationalism” to distinguish it and oppose it to Anglo-Saxon empiricism (Hobbes, Locke, Hume, etc.).

We will follow here a terminology distinguishing a modern rationalism (from Descartes to Leibniz), from a critical rationalism to generally designate Kantian and post-Kantian rationalism, independently of the nuances, sometimes sensitive, of which it is composed.

The word rationalism was also used before the Renaissance, and during the Middle Ages: it was then rationalism in theology.

Sources in ancient Greece

The intellectual attitude aimed at placing reason and rational procedures as sources of knowledge dates back to ancient Greece, when under the name of logos (which originally meant speech), it broke away from mythical thought and, from the sciences, gives birth to philosophy.

Plato sees in sensitivity only a pseudo knowledge giving access only to the sensitive, material and changing reality of the world. To rely on sensory experience is to be like prisoners locked up in a cave who take the shadows passing on the dimly lit wall for reality itself. “Let no one enter here if he is not a geometrician”, he had engraved on the pediment of his school: the exercise of mathematics teaches us to detach ourselves from our senses and to exercise our only reason, a necessary prerequisite for philosophical dialectic. Knowledge of the real is knowledge of Ideas or essences, intelligible and immutable realities, and this knowledge is rational. In this sense there is a Platonic rationalism.

Aristotle, on the contrary, bases his philosophy on the concrete observation of nature (physis), and lays the foundations:

  • formal logic, in his Organon (we would call it general logic today),
  • of what was later called metaphysics (beyond physis, that is to say, beyond nature),
  • of ethics (Nicomachean Ethics).

Modern rationalism

Origin and characteristics of modern rationalism

It is not the use of reason, nor its claim, which suffices to define rationalism as a doctrine. Modern rationalism is constituted and systematized at the end of the Renaissance, within the framework of the Ptolemaic-Copernican controversy, which leads to the mathematization of physics. After the trial of Galileo (1633), and reinforced in the project of reforming philosophy which Cardinal de Bérulle had made him an “obligation of conscience” a few years earlier, René Descartes concretized his project by publishing several works of philosophy, in particular the famous Discourse on Method (1637), and the Meditations on First Philosophy (1641). Descartes, with his Cogito ergo sum, thus becomes one of the main representatives of modern rationalism.

Modern rationalism rests on the metaphysical postulate that the principles underlying reality are identical to the laws of reason itself. So it is with the principle of determining reason (or sufficient reason) which Leibniz, in the Essays on Theodicy (1710), states that it is that nothing ever happens, without there being a cause or at least a determining reason, that is to say, something that can serve to explain a priori why it exists rather than non-existent, and why this is so rather than any other way.

Since nothing exists or happens without a cause, there is therefore nothing that is not, in principle, intelligible and explicable by reason. Within the framework of onto-theology, this identity of thought and being finds its ultimate justification in God, creator of the world and its laws on the one hand, of human reason and its principles on the other. That in which rationalism thus understood is fully accomplished in philosophical idealism, to which Hegel will give its most systematic form, in the formula: “what is rational is effective, and what is effective is rational” (Preface to Elements of the Philosophy of Right).

It follows that reason, containing universal principles and a priori ideas expressing eternal truths, is immutable and identical in every man. It is in this sense that Descartes, in the Discourse on Method, writes: “Common sense is the best shared thing in the world”, specifying that “the power to judge well and to distinguish the true from the false, which is properly what is called good sense or reason, is naturally equal in all men.”

From the point of view of the origin of our knowledge, rationalism is traditionally opposed to empiricism, irrationalism, and revelation.

Rationalism and empiricism

According to empiricism, experience is the source of all our knowledge. As John Locke explains in the 1690 An Essay Concerning Human Understanding:

“Let us then suppose the mind to be, as we say, white paper, void of all characters, without any ideas: how comes it to be furnished? … To this I answer, in one word, from experience. In that all our knowledge is founded; and from that it ultimately derives itself.”

This experience is that of our external senses, which allows us for example to form the idea of ​​color, but also that of our thought in action, by which we are able to form the idea of ​​thought, or of reasoning.

Such a position leads to a devaluation of reason: an idea is, in the eyes of David Hume, only a copy of an analogous impression, so that all this creative power of the mind is nothing more than the faculty of combining, transposing, diminishing the materials provided by the senses and experience (An Enquiry Concerning Human Understanding, 1748), combinations which he operates according to relations of resemblance or contiguity. From the point of view of empiricism therefore, there is nothing in the intellect which was not first in the sensibility, to which Leibniz would retort except the intellect itself.

Rationalism postulates, in fact, the existence in reason of universal logical principles (principle of the excluded middle, principle of sufficient reason) and a priori ideas, that is to say, ideas independent of experience and preceding any experience. Thus Descartes admits the existence of a priori and innate ideas such as the idea of ​​infinity, of time, of number, or the very idea of ​​God who is “like the mark of the worker on his work “, simple and primary ideas, without which the sensible experience would remain unintelligible to us: “I consider that there are in us certain primitive notions, which are like originals, on the pattern of which we form all our other knowledge “(Letter to Elisabeth of May 21, 1643).

In the eyes of rationalism, in fact, sensory experience cannot give true knowledge. Plato already denounced its fluctuating and relative character, which shows us only an inconsistent play of shadows, and Descartes, in the first Meditations on First Philosophy, its deceptive character:

“All that I have hitherto received for the most true and certain, I have learned from the senses or by the senses: now I have sometimes found these senses to be deceptive, and it is prudent to never fully trusting those who have once deceived us.”

Rationalism, however, as we will see below in its critical form, does not repudiate sensory experience but subjects it to a priori forms which make it possible and organize its given.

Rationalism and irrationalism

We must understand here by irrationalism the reference to any experience or any faculty other than reason and not obeying its laws, supposed to give a deeper and more authentic knowledge of phenomena and beings, and leaving room for a fringe of ineffable, mystery, or inexplicable. In this sense, rationalism is opposed to mysticism, magic, the occult, sentimentalism, the paranormal or even superstition. Only rational processes are authoritative: intellectual evidence, demonstration, reasoning.

It is romantic sentimentalism that Hegel attacks in the preface to the Phenomenology of Spirit, when he evokes this so-called philosophy which “by the absence of concept presents itself as an intuitive and poetic thought, throws on the market for fanciful combinations, of a fantasy only disorganized by thought – fancies which are neither flesh nor fish, neither poetry nor philosophy”. He who claims to touch the truth in the ineffable experience of intimate feeling condemns himself to silence and the solitude of incommunicability; “in other words, he tramples on the root of humanity.”

Rationalism and revelation

From a theological perspective, rationalism emphasizes the natural light of reason as opposed to the revealed knowledge of faith. Against fideism, he demands that the articles of faith and the Scriptures themselves be subjected to rational scrutiny.

Spinoza, in the Theological-Political Treatise, develops a critical reading of the Old Testament.

Descartes, in the preface “to the Deans and Doctors of the Faculty of Theology of Paris” which precedes the Meditations on First Philosophy, asserts that “everything that can be known about God can be shown by reasons that are not necessary to seek elsewhere than within ourselves, and which our mind alone is capable of supplying us”. According to him, in fact, parallel to revealed theology, the only reason allows us to demonstrate the existence of God by the ontological argument, so that the existence of God necessarily derives from his essence, as it necessarily derives from the essence of the triangle that the sum of its angles equals two right angles.”

Thus, for Descartes, the search for truth can be done by reason alone, without the light of faith (Principles of Philosophy). The cogito ergo sum postulates that man is an intelligent substance that can access the truth.

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