,
The Sleep of Reason Produces Monsters (), The proposal that reason gives humanity a special position in nature has been argued to be a defining characteristic of
Western philosophy and later Western
science, starting with classical Greece. Philosophy can be described as a way of life based upon reason, while reason has been among the major subjects of philosophical discussion since ancient times. Reason is often said to be
reflexive, or "self-correcting", and the critique of reason has been a persistent theme in philosophy.
Classical philosophy For many classical
philosophers, nature was understood
teleologically, meaning that every type of thing had a definitive purpose that fit within a natural order that was itself understood to have aims. Perhaps starting with
Pythagoras or
Heraclitus, the
cosmos was even said to have reason. Reason, by this account, is not just a characteristic that people happen to have. Reason was considered of higher stature than other characteristics of human nature, because it is something people share with nature itself, linking an apparently immortal part of the human mind with the divine order of the cosmos. Within the human
mind or
soul (), reason was described by
Plato as being the natural monarch which should rule over the other parts, such as spiritedness () and the passions.
Aristotle, Plato's student, defined human beings as
rational animals, emphasizing reason as a characteristic of
human nature. He described the highest human happiness or well being () as a life which is lived consistently, excellently, and completely in accordance with reason. The conclusions to be drawn from the discussions of Aristotle and Plato on this matter are amongst the most debated in the history of philosophy. But teleological accounts such as Aristotle's were highly influential for those who attempt to explain reason in a way that is consistent with
monotheism and the immortality and divinity of the human soul. For example, in the
neoplatonist account of
Plotinus, the
cosmos has one soul, which is the seat of all reason, and the souls of all people are part of this soul. Reason is for Plotinus both the provider of form to material things, and the light which brings people's souls back into line with their source.
Christian and Islamic philosophy The classical view of reason was adopted by the early Church. The greatest among the early
Church Fathers and
Doctors of the Church such as
Augustine of Hippo,
Basil of Caesarea, and
Gregory of Nyssa were as much Neoplatonic philosophers as they were Christian theologians, and they adopted the Neoplatonic view of human reason and its implications for our relationship to creation, to ourselves, and to God. The Neoplatonic conception of the rational aspect of the human soul was widely adopted by medieval Islamic philosophers and continues to hold significance in
Iranian philosophy. Among the Scholastics who relied on the classical concept of reason for the development of their doctrines, none were more influential than
Saint Thomas Aquinas, who put this concept at the heart of his
Natural Law. In this doctrine, Thomas concludes that because humans have reason and because reason is a spark of the divine, every single human life is invaluable, all humans are equal, and every human is born with an intrinsic and permanent set of basic rights. On this foundation, the idea of human rights would later be constructed by Spanish theologians at the
School of Salamanca. Other Scholastics, such as
Roger Bacon and
Albertus Magnus, following the example of Islamic scholars such as
Alhazen, emphasised reason an intrinsic human ability to decode the created order and the structures that underlie our experienced physical reality. This interpretation of reason was instrumental to the development of the scientific method in the early Universities of the high Middle Ages.
Subject-centred reason in early modern philosophy The
early modern era was marked by a number of significant changes in the understanding of reason, starting in
Europe. One of the most important of these changes involved a change in the
metaphysical understanding of human beings. Scientists and philosophers began to question the
teleological understanding of the world. Nature was no longer assumed to be human-like, with its own aims or reason, and human nature was no longer assumed to work according to anything other than the same "
laws of nature" which affect inanimate things. This new understanding eventually displaced the previous
world view that derived from a spiritual understanding of the universe. Accordingly, in the 17th century,
René Descartes explicitly rejected the traditional notion of humans as "rational animals", suggesting instead that they are nothing more than "thinking things" along the lines of other "things" in nature. Any grounds of knowledge outside that understanding was, therefore, subject to doubt. In his search for a foundation of all possible knowledge, Descartes brought into doubt
all knowledge—
except that of the mind itself in the process of thinking: At this time I admit nothing that is not necessarily true. I am therefore precisely nothing but a thinking thing; that is a mind, or intellect, or understanding, or reason—words of whose meanings I was previously ignorant. This eventually became known as
epistemological or "subject-centred" reason, because it is based on the
knowing subject, who perceives the rest of the world and itself as a set of objects to be studied, and successfully mastered, by applying the knowledge accumulated through such study. Breaking with tradition and with many thinkers after him, Descartes explicitly did not divide the incorporeal soul into parts, such as reason and intellect, describing them instead as one indivisible incorporeal entity. A contemporary of Descartes,
Thomas Hobbes described reason as a broader version of "addition and subtraction" which is not limited to numbers. This understanding of reason is sometimes termed "calculative" reason. Similar to Descartes, Hobbes asserted that "No discourse whatsoever, can end in absolute knowledge of fact, past, or to come" but that "sense and memory" is absolute knowledge. In the late 17th century through the 18th century,
John Locke and
David Hume developed Descartes's line of thought still further. Hume took it in an especially
skeptical direction, proposing that there could be no possibility of
deducing relationships of cause and effect, and therefore no knowledge is based on reasoning alone, even if it seems otherwise. Hume famously remarked that, "We speak not strictly and philosophically when we talk of the combat of
passion and of reason. Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them." Hume also took his definition of reason to unorthodox extremes by arguing, unlike his predecessors, that human reason is not qualitatively different from either simply conceiving individual ideas, or from judgments associating two ideas, and that "reason is nothing but a wonderful and unintelligible instinct in our souls, which carries us along a certain train of ideas, and endows them with particular qualities, according to their particular situations and relations." It followed from this that animals have reason, only much less complex than human reason. In the 18th century,
Immanuel Kant attempted to show that Hume was wrong by demonstrating that a "
transcendental" self, or "I", was a necessary condition of all experience. Therefore, suggested Kant, on the basis of such a self, it is in fact possible to reason both about the conditions and limits of human knowledge. And so long as these limits are respected, reason can be the vehicle of morality, justice, aesthetics, theories of knowledge (
epistemology), and understanding.
Substantive and formal reason In the formulation of Kant, who wrote some of the most influential modern treatises on the subject, the great achievement of reason () is that it is able to exercise a kind of universal law-making. Kant was able therefore to reformulate the basis of moral-practical, theoretical, and aesthetic reasoning on "universal" laws. Here,
practical reasoning is the self-legislating or self-governing formulation of universal
norms, and
theoretical reasoning is the way humans posit universal
laws of nature. Under practical reason, the moral
autonomy or freedom of people depends on their ability, by the proper exercise of that reason, to behave according to laws that are given to them. This contrasted with earlier forms of morality, which depended on
religious understanding and interpretation, or on
nature, for their substance. According to Kant, in a free society each individual must be able to pursue their goals however they see fit, as long as their actions conform to principles given by reason. He formulated such a principle, called the "
categorical imperative", which would justify an action only if it could be universalized: Act only according to that maxim whereby you can, at the same time, will that it should become a universal law. In contrast to Hume, Kant insisted that reason itself (German ) could be used to find solutions to metaphysical problems, especially the discovery of the foundations of morality. Kant claimed that these solutions could be found with his "
transcendental logic", which unlike normal logic is not just an instrument that can be used indifferently, as it was for Aristotle, but a theoretical science in its own right and the basis of all the others. According to
Jürgen Habermas, the "substantive unity" of reason has dissolved in modern times, such that it can no longer answer the question "How should I live?" Instead, the unity of reason has to be strictly formal, or "procedural". He thus described reason as a group of three autonomous spheres (on the model of Kant's three critiques): ; Cognitive–instrumental reason: the kind of reason employed by the sciences; used to observe events, to predict and control outcomes, and to intervene in the world on the basis of its hypotheses ; Moral–practical reason: what we use to deliberate and discuss issues in the moral and political realm, according to universalizable procedures (similar to Kant's categorical imperative) ; Aesthetic reason: typically found in works of art and literature, and encompasses the novel ways of seeing the world and interpreting things that those practices embody For Habermas, these three spheres are the domain of experts, and therefore need to be mediated with the "
lifeworld" by philosophers. In drawing such a picture of reason, Habermas hoped to demonstrate that the substantive unity of reason, which in pre-modern societies had been able to answer questions about the good life, could be made up for by the unity of reason's formalizable procedures.
The critique of reason Hamann,
Herder,
Kant,
Hegel,
Kierkegaard,
Nietzsche,
Heidegger,
Foucault,
Rorty, and many other philosophers have contributed to a debate about what reason means, or ought to mean. Some, like Kierkegaard, Nietzsche, and Rorty, are skeptical about subject-centred, universal, or instrumental reason, and even skeptical toward reason as a whole. Others, including Hegel, believe that it has obscured the importance of
intersubjectivity, or "spirit" in human life, and they attempt to reconstruct a model of what reason should be. Some thinkers, e.g. Foucault, believe there are other
forms of reason, neglected but essential to modern life, and to our understanding of what it means to live a life according to reason. In the last several decades, a number of proposals have been made to "re-orient" this critique of reason, or to recognize the "other voices" or "new departments" of reason: For example, in opposition to subject-centred reason, Habermas has proposed a model of
communicative reason that sees it as an essentially cooperative activity, based on the fact of linguistic
intersubjectivity.
Nikolas Kompridis proposed a widely encompassing view of reason as "that ensemble of practices that contributes to the opening and preserving of openness" in human affairs, and a focus on reason's possibilities for social change. The philosopher
Charles Taylor, influenced by the 20th century German philosopher
Martin Heidegger, proposed that reason ought to include the faculty of
disclosure, which is tied to the way we make sense of things in everyday life, as a new "department" of reason. In the essay "What is Enlightenment?", Michel Foucault proposed a critique based on Kant's distinction between "private" and "public" uses of reason: ; Private reason : the reason that is used when an individual is "a cog in a machine" or when one "has a role to play in society and jobs to do: to be a soldier, to have taxes to pay, to be in charge of a parish, to be a civil servant" ; Public reason : the reason used "when one is reasoning as a reasonable being (and not as a cog in a machine), when one is reasoning as a member of reasonable humanity"; in these circumstances, "the use of reason must be free and public" ==Reason compared to related concepts==