Rationalist philosophy in Western antiquity '' by
Raphael.
Vatican Palace,
Vatican City Although rationalism in its modern form post-dates antiquity, philosophers from this time laid down the foundations of rationalism. In particular, the understanding that we may be aware of knowledge available only through the use of rational thought.
Pythagoras (570–495 BCE) Pythagoras was one of the first Western philosophers to stress rationalist insight. He is often revered as a great
mathematician,
mystic and
scientist, but he is best known for the
Pythagorean theorem, which bears his name, and for discovering the mathematical relationship between the length of strings on lute and the pitches of the notes. Pythagoras "believed these harmonies reflected the ultimate nature of reality. He summed up the implied metaphysical rationalism in the words 'All is number'. It is probable that he had caught the rationalist's vision, later seen by
Galileo (1564–1642), of a world governed throughout by mathematically formulable laws". It has been said that he was the first man to call himself a philosopher, or lover of wisdom.
Plato (427–347 BCE) in
The School of Athens, by
Raphael Plato held rational insight to a very high standard, as is seen in his works such as
Meno and
The Republic. He taught on the
Theory of Forms (or the Theory of Ideas) which asserts that the highest and most fundamental kind of reality is not the material world of change
known to us through sensation, but rather the abstract, non-material (but
substantial) world of forms (or ideas). For Plato, these forms were accessible only to reason and not to sense.
Aristotle (384–322 BCE) Aristotle's main contribution to rationalist thinking was the use of
syllogistic logic and its use in argument. Aristotle defines syllogism as "a discourse in which certain (specific) things having been supposed, something different from the things supposed results of necessity because these things are so." Despite this very general definition, Aristotle limits himself to categorical syllogisms which consist of three
categorical propositions in his work
Prior Analytics. These included categorical
modal syllogisms.
Middle Ages Portrait on Silver Vase Although the three great Greek philosophers disagreed with one another on specific points, they all agreed that rational thought could bring to light knowledge that was self-evidentinformation that humans otherwise could not know without the use of reason. After Aristotle's death, Western rationalistic thought was generally characterized by its application to theology, such as in the works of
Augustine, the
Islamic philosopher Avicenna (Ibn Sina),
Averroes (Ibn Rushd), and Jewish philosopher and theologian
Maimonides. The
Waldensians sect also incorporated rationalism into their movement. One notable event in the Western timeline was the philosophy of
Thomas Aquinas who attempted to merge Greek rationalism and Christian revelation in the thirteenth-century. Generally, the
Roman Catholic Church viewed Rationalists as a threat, labeling them as those who "while admitting revelation, reject from the word of God whatever, in their private judgment, is inconsistent with human reason."
Classical rationalism René Descartes (1596–1650) Descartes was the first of the modern rationalists and has been dubbed the 'Father of Modern Philosophy.' Much subsequent
Western philosophy is a response to his writings, which are studied closely to this day. Descartes thought that only knowledge of eternal truthsincluding the truths of mathematics, and the epistemological and metaphysical foundations of the sciences could be attained by reason alone; other knowledge, the knowledge of physics, required experience of the world, aided by the
scientific method. He also argued that although
dreams appear as real as
sense experience, these dreams cannot provide persons with knowledge. Also, since conscious sense experience can be the cause of illusions, then sense experience itself can be doubtable. As a result, Descartes deduced that a rational pursuit of truth should doubt every belief about sensory reality. He elaborated these beliefs in such works as
Discourse on the Method,
Meditations on First Philosophy, and
Principles of Philosophy. Descartes developed a method to attain truths according to which nothing that cannot be recognised by the intellect (or
reason) can be classified as knowledge. These truths are gained "without any sensory experience", according to Descartes. Truths that are attained by reason are broken down into elements that intuition can grasp, which, through a purely deductive process, will result in clear truths about reality. Descartes therefore argued, as a result of his method, that reason alone determined knowledge, and that this could be done independently of the senses. For instance, his famous dictum,
cogito ergo sum or "I think, therefore I am", is a conclusion reached
a priori i.e., prior to any kind of experience on the matter. The simple meaning is that doubting one's existence, in and of itself, proves that an "I" exists to do the thinking. In other words, doubting one's own doubting is absurd. Spinoza's philosophy is a system of ideas constructed upon basic building blocks with an internal consistency with which he tried to answer life's major questions and in which he proposed that "God exists only philosophically."
Euclid and
Thomas Hobbes, and much intellectual attention.
Gottfried Leibniz (1646–1716) Leibniz was the last major figure of seventeenth-century rationalism who contributed heavily to other fields such as
metaphysics,
epistemology,
logic,
mathematics,
physics,
jurisprudence, and the
philosophy of religion; he is also considered to be one of the last "universal geniuses". He did not develop his system, however, independently of these advances. Leibniz rejected Cartesian dualism and denied the existence of a material world. In Leibniz's view there are infinitely many simple substances, which he called "
monads" (which he derived directly from
Proclus). Leibniz developed his theory of monads in response to both Descartes and
Spinoza, because the rejection of their visions forced him to arrive at his own solution. Monads are the fundamental unit of reality, according to Leibniz, constituting both inanimate and animate objects. These units of reality represent the universe, though they are not subject to the laws of causality or space (which he called "
well-founded phenomena"). Leibniz, therefore, introduced his principle of
pre-established harmony to account for apparent causality in the world.
Immanuel Kant (1724–1804) Kant is one of the central figures of modern
philosophy, and set the terms by which all subsequent thinkers have had to grapple. He argued that human perception structures natural laws, and that reason is the source of morality. His thought continues to hold a major influence in contemporary thought, especially in fields such as metaphysics, epistemology, ethics, political philosophy, and aesthetics. Kant named his brand of epistemology "
Transcendental Idealism", and he first laid out these views in his famous work
The Critique of Pure Reason. In it he argued that there were fundamental problems with both rationalist and empiricist dogma. To the rationalists he argued, broadly, that pure reason is flawed when it goes beyond its limits and claims to know those things that are necessarily beyond the realm of every possible experience: the
existence of God, free will, and the immortality of the human soul. Kant referred to these objects as "The Thing in Itself" and goes on to argue that their status as objects beyond all possible experience by definition means we cannot know them. To the empiricist, he argued that while it is correct that experience is fundamentally necessary for human knowledge, reason is necessary for processing that experience into coherent thought. He therefore concludes that both reason and experience are necessary for human knowledge. In the same way, Kant also argued that it was wrong to regard thought as mere analysis. "In Kant's views,
a priori concepts do exist, but if they are to lead to the amplification of knowledge, they must be brought into relation with empirical data".
Contemporary rationalism Rationalism has become a rarer label of philosophers today; rather many different kinds of specialised rationalisms are identified. For example,
Robert Brandom has appropriated the terms "rationalist expressivism" and "rationalist pragmatism" as labels for aspects of his programme in
Articulating Reasons, and identified "linguistic rationalism", the claim that the contents of propositions "are essentially what can serve as both premises and conclusions of inferences", as a key thesis of
Wilfred Sellars. Outside of academic philosophy, some participants in the internet communities surrounding
LessWrong and
Slate Star Codex have described themselves as "rationalists" or the "
rationalist community" in reference to
rationality, rather than rationalism. The term has also been used in this way by critics such as
Timnit Gebru. ==Criticism==