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philosophy Philosophy (from , ) is the systematized study of general and fundamental questions, such as those about existence, reason, knowledge, values, mind, and language. Such questions are often posed as problems to be studied or resolved. Some ...
, a noumenon (, ; ; noumena) is a posited
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...
or an
event Event may refer to: Gatherings of people * Ceremony, an event of ritual significance, performed on a special occasion * Convention (meeting), a gathering of individuals engaged in some common interest * Event management, the organization of eve ...
that exists independently of human
sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of Stimulus (physiology), stimuli. (For example, in the human body, the brain which is part of the cen ...
and/or
perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...
. The term ''noumenon'' is generally used in contrast with, or in relation to, the term ''
phenomenon A phenomenon ( : phenomena) is an observable event. The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be directly observed. Kant was heavily influenced by Gottfried W ...
'', which refers to any
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...
of the senses.
Immanuel Kant Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher and one of the central Enlightenment thinkers. Born in Königsberg, Kant's comprehensive and systematic works in epistemology, metaphysics, ethics, and ...
first developed the notion of the noumenon as part of his
transcendental idealism Transcendental idealism is a philosophical system founded by German philosopher Immanuel Kant in the 18th century. Kant's epistemological program is found throughout his '' Critique of Pure Reason'' (1781). By ''transcendental'' (a term that des ...
, suggesting that while we know the noumenal world to exist because human
sensibility Sensibility refers to an acute perception of or responsiveness toward something, such as the emotions of another. This concept emerged in eighteenth-century Britain, and was closely associated with studies of sense perception as the means thro ...
is merely receptive, it is not itself sensible and must therefore remain otherwise
unknowable Epistemology (; ), or the theory of knowledge, is the branch of philosophy concerned with knowledge. Epistemology is considered a major subfield of philosophy, along with other major subfields such as ethics, logic, and metaphysics. Episte ...
to us. In
Kantian philosophy Kantianism is the philosophy of Immanuel Kant, a German philosopher born in Königsberg, Prussia (now Kaliningrad, Russia). The term ''Kantianism'' or ''Kantian'' is sometimes also used to describe contemporary positions in philosophy of mind, ...
, the unknowable noumenon is often identified with or associated with the unknowable "
thing-in-itself In Kantian philosophy, the thing-in-itself (german: Ding an sich) is the status of objects as they are, independent of representation and observation. The concept of the thing-in-itself was introduced by the German philosopher Immanuel Kant, and ...
" (german: Ding an sich). However, the nature of the relationship between the two is not made explicit in Kant's work, and remains a subject of debate among Kant scholars as a result.


Etymology

The
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
word grc, νοούμενoν, nooúmenon, label=none (plural grc, νοούμενα, nooúmena, label=none) is the neuter middle-passive present participle of grc, νοεῖν, noeîn, to think, to mean, label=none, which in turn originates from the word grc, νοῦς,
noûs ''Noûs'' is a quarterly peer-reviewed academic journal on philosophy published by Wiley-Blackwell. It was established in 1967 by Hector-Neri Castañeda and is currently edited by Ernest Sosa (Rutgers University). The journal is accompanied by t ...
, label=none, an
Attic An attic (sometimes referred to as a '' loft'') is a space found directly below the pitched roof of a house or other building; an attic may also be called a ''sky parlor'' or a garret. Because attics fill the space between the ceiling of the ...
contracted form of grc, νόος, nóos, perception, understanding, mind, label=none. A rough equivalent in English would be "something that is thought", or "the object of an act of thought".


Historical predecessors

Regarding the equivalent concepts in
Plato Plato ( ; grc-gre, Πλάτων ; 428/427 or 424/423 – 348/347 BC) was a Greek philosopher born in Athens during the Classical period in Ancient Greece. He founded the Platonist school of thought and the Academy, the first institution ...
, Ted Honderich writes: " Platonic Ideas and Forms are noumena, and phenomena are things displaying themselves to the senses... This dichotomy is the most characteristic feature of Plato's dualism; that noumena and the noumenal world are objects of the highest knowledge, truths, and values is Plato's principal legacy to philosophy." However, that noumena and the noumenal world were objects of the highest knowledge, truths, and values, was disputed from the start, beginning with
Democritus Democritus (; el, Δημόκριτος, ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greek pre-Socratic philosopher from Abdera, primarily remembered today for his formulation of an atomic theory of the universe. No ...
, his follower
Pyrrho Pyrrho of Elis (; grc, Πύρρων ὁ Ἠλεῖος, Pyrrhо̄n ho Ēleios; ), born in Elis, Greece, was a Greek philosopher of Classical antiquity, credited as being the first Greek skeptic philosopher and founder of Pyrrhonism. Life ...
, founder of
Pyrrhonism Pyrrhonism is a school of philosophical skepticism founded by Pyrrho in the fourth century BCE. It is best known through the surviving works of Sextus Empiricus, writing in the late second century or early third century CE. History Pyrrho of E ...
, and even in the
Academy An academy ( Attic Greek: Ἀκαδήμεια; Koine Greek Ἀκαδημία) is an institution of secondary or tertiary higher learning (and generally also research or honorary membership). The name traces back to Plato's school of philosophy ...
starting with
Arcesilaus Arcesilaus (; grc-gre, Ἀρκεσίλαος; 316/5–241/0 BC) was a Greek Hellenistic philosopher. He was the founder of Academic Skepticism and what is variously called the Second or Middle or New Academy – the phase of the Platonic Acade ...
and the introduction of
Academic Skepticism Academic skepticism refers to the skeptical period of ancient Platonism dating from around 266 BCE, when Arcesilaus became scholarch of the Platonic Academy, until around 90 BCE, when Antiochus of Ascalon rejected skepticism, although individua ...
.


Kantian noumena


Overview

As expressed in Kant's '' Critique of Pure Reason,'' human understanding is structured by "concepts of the understanding" or pure
categories of understanding In Immanuel Kant's philosophy, a category (german: Categorie in the original or ''Kategorie'' in modern German) is a pure concept of the understanding (''Verstand''). A Kantian category is a characteristic of the appearance of any object in gen ...
, found prior to experience in the
mind The mind is the set of faculties responsible for all mental phenomena. Often the term is also identified with the phenomena themselves. These faculties include thought, imagination, memory, will, and sensation. They are responsible for various m ...
and which make outer experiences possible as counterpart to the
rational Rationality is the quality of being guided by or based on reasons. In this regard, a person acts rationally if they have a good reason for what they do or a belief is rational if it is based on strong evidence. This quality can apply to an abili ...
faculties of the mind. By Kant's account, when one employs a concept to describe or categorize ''noumena'' (the objects of inquiry, investigation or analysis of the workings of the world), one is also employing a way of describing or categorizing ''phenomena'' (the observable manifestations of those objects of inquiry, investigation or analysis). Kant posited methods by which human understanding makes sense of and thus intuits phenomena that appear to the mind: the concepts of the '' transcendental aesthetic'', as well as that of the ''transcendental analytic'', '' transcendental logic'' and ''transcendental deduction''.''The Encyclopedia of Philosophy'' (Macmillan, 1967, 1996) Volume 4, "Kant, Immanuel", section on "Critique of Pure Reason: Theme and Preliminaries", p. 308 ''ff''.''The Encyclopedia of Philosophy'' (Macmillan, 1967, 1996) Volume 4, "Kant, Immanuel", section on "Transcendental Aesthetic", p. 310 ''ff''.''The Encyclopedia of Philosophy'' (Macmillan, 1967, 1996) Volume 4, "Kant, Immanuel", section on "Pure Concepts of the Understanding", p. 311 ''ff''. Taken together, Kant's "categories of understanding" are the principles of the human mind which necessarily are brought to bear in attempting to understand the world in which we
exist eXist-db (or eXist for short) is an open source software project for NoSQL databases built on XML technology. It is classified as both a NoSQL document-oriented database system and a native XML database (and it provides support for XML, JSON, HTM ...
(that is, to understand, or attempt to understand, "things in themselves"). In each instance the word "transcendental" refers to the process that the human mind must exercise to understand or grasp the form of, and order among, phenomena. Kant asserts that to "transcend" a direct observation or experience is to use reason and classifications to strive to correlate with the phenomena that are observed. Humans can make sense out of phenomena in these various ways, but in doing so can never know the "things-in-themselves", the actual objects and dynamics of the natural world in their noumenal dimension - this being the negative, correlate to phenomena and that which escapes the limits of human understanding. By Kant's Critique, our minds may attempt to correlate in useful ways, perhaps even closely accurate ways, with the structure and order of the various aspects of the universe, but cannot know these "things-in-themselves" (noumena) directly. Rather, we must infer the extent to which the human rational faculties can reach the object of "things-in-themselves" by our observations of the manifestations of those things that can be perceived via the physical senses, that is, of phenomena, and by ordering these perceptions in the mind help infer the validity of our perceptions to the rational categories used to understand them in a rational system. This rational system (''transcendental analytic''), being the categories of the understanding as free from empirical contingency. According to Kant, objects of which we are cognizant via the physical senses are merely representations of ''unknown somethings''—what Kant refers to as the ''transcendental object''—as interpreted through the ''
a priori ("from the earlier") and ("from the later") are Latin phrases used in philosophy to distinguish types of knowledge, justification, or argument by their reliance on empirical evidence or experience. knowledge is independent from current ex ...
'' or ''categories of the understanding''. These ''unknown somethings'' are manifested within the noumenon—although we can never know how or why as our perceptions of these ''unknown somethings'' via our physical senses are bound by the limitations of the ''categories of the understanding'' and we are therefore never able to fully know the "thing-in-itself".


Noumenon and the thing-in-itself

Many accounts of Kant's philosophy treat "noumenon" and "thing-in-itself" as synonymous, and there is textual evidence for this relationship. However,
Stephen Palmquist Stephen Richard Palmquist (born 1957) is an American philosopher, currently living in Los Angeles. He taught philosophy at various universities in Hong Kong from 1987 to 2021. A Patheos article referred to him as "one of the greatest living inter ...
holds that "noumenon" and "thing-in-itself" are only ''loosely'' synonymous, inasmuch as they represent the same concept viewed from two different perspectives, and other scholars also argue that they are not identical. Schopenhauer criticised Kant for changing the meaning of "noumenon". However, this opinion is far from unanimous. Kant's writings show points of difference between noumena and things-in-themselves. For instance, he regards things-in-themselves as existing:
...though we cannot know these objects as things in themselves, we must yet be in a position at least to think them as things in themselves; otherwise we should be landed in the absurd conclusion that there can be appearance without anything that appears.
He is much more doubtful about noumena:
But in that case a noumenon is not for our understanding a special
ind of Ind or IND may refer to: General * Independent (politician), a politician not affiliated to any political party * Independent station, used within television program listings and the television industry for a station that is not affiliated with ...
object, namely, an intelligible object; the ort ofunderstanding to which it might belong is itself a problem. For we cannot in the least represent to ourselves the possibility of an understanding which should know its object, not discursively through categories, but intuitively in a non-sensible intuition.
A crucial difference between the noumenon and the thing-in-itself is that to call something a noumenon is to claim a kind of knowledge, whereas Kant insisted that the thing-in-itself is unknowable. Interpreters have debated whether the latter claim makes sense: it seems to imply that we know at least one thing about the thing-in-itself (i.e., that it is unknowable). But Stephen Palmquist explains that this is part of Kant's definition of the term, to the extent that anyone who claims to have found a way of making the thing-in-itself knowable must be adopting a non-Kantian position.


Positive and negative noumena

Kant also makes a distinction between ''positive'' and ''negative'' noumena:
If by 'noumenon' we mean a thing so far as it is not an object of our sensible intuition, and so abstract from our mode of intuiting it, this is a noumenon in the ''negative'' sense of the term.
But if we understand by it an object of a non-sensible intuition, we thereby presuppose a special mode of intuition, namely, the intellectual, which is not that which we possess, and of which we cannot comprehend even the possibility. This would be 'noumenon' in the ''positive'' sense of the term.
The positive noumena, if they existed, would be immaterial entities that can only be apprehended by a special, non-sensory faculty: "intellectual intuition" (''nicht sinnliche
Anschauung Anschauung is a German concept that is usually translated as "intuition". It, however, connotes a more nuanced definition especially when the concept is applied to philosophical discourse, including quantum theory. Some of the translations include a ...
''). Kant doubts that we have such a faculty, because for him intellectual intuition would mean that thinking of an entity, and its being represented, would be the same. He argues that humans have no way to apprehend positive noumena:
Since, however, such a type of intuition, intellectual intuition, forms no part whatsoever of our faculty of knowledge, it follows that the employment of the categories can never extend further than to the objects of experience. Doubtless, indeed, there are intelligible entities corresponding to the sensible entities; there may also be intelligible entities to which our sensible faculty of intuition has no relation whatsoever; but our concepts of understanding, being mere forms of thought for our sensible intuition, could not in the least apply to them. That, therefore, which we entitle 'noumenon' must be understood as being such only in a negative sense.


The noumenon as a limiting concept

Even if noumena are unknowable, they are still needed as a ''limiting concept'', Kant tells us. Without them, there would be only phenomena, and since potentially we have complete knowledge of our phenomena, we would in a sense know everything. In his own words:
Further, the concept of a noumenon is necessary, to prevent sensible intuition from being extended to things in themselves, and thus to limit the objective validity of sensible knowledge.
What our understanding acquires through this concept of a noumenon, is a negative extension; that is to say, understanding is not limited through sensibility; on the contrary, it itself limits sensibility by applying the term noumena to things in themselves (things not regarded as appearances). But in so doing it at the same time sets limits to itself, recognising that it cannot know these noumena through any of the categories, and that it must therefore think them only under the title of an unknown something.
Furthermore, for Kant, the existence of a noumenal world limits reason to what he perceives to be its proper bounds, making many questions of traditional metaphysics, such as the existence of God, the soul, and free will unanswerable by reason. Kant derives this from his
definition of knowledge Definitions of knowledge try to determine the essential features of knowledge. Closely related terms are conception of knowledge, theory of knowledge, and analysis of knowledge. Some general features of knowledge are widely accepted among philoso ...
as "the determination of given representations to an object". As there are no appearances of these entities in the phenomenal, Kant is able to make the claim that they cannot be known to a mind that works upon "such knowledge that has to do only with appearances". These questions are ultimately the "proper object of faith, but not of reason".


The dual-object and dual-aspect interpretations

Kantian scholars have long debated two contrasting interpretations of the thing-in-itself. One is the ''dual object'' view, according to which the thing-in-itself is an entity distinct from the phenomena to which it gives rise. The other is the ''dual aspect'' view, according to which the thing-in-itself and the thing-as-it-appears are two "sides" of the same thing. This view is supported by the textual fact that "Most occurrences of the phrase 'things-in-themselves' are shorthand for the phrase, 'things considered in themselves' (Dinge an sich selbst betrachtet)." Although we cannot ''see'' things apart from the way we do in fact perceive them via the physical senses, we can ''think'' them apart from our mode of sensibility (physical perception); thus making the thing-in-itself a kind of noumenon or object of thought.


Criticisms of Kant's noumenon


Pre-Kantian critique

Though the term ''noumenon'' did not come into common usage until Kant, the idea that undergirds it, that matter has an absolute existence which causes it to emanate certain phenomena, had historically been subjected to criticism.
George Berkeley George Berkeley (; 12 March 168514 January 1753) – known as Bishop Berkeley (Bishop of Cloyne of the Anglican Church of Ireland) – was an Anglo-Irish philosopher whose primary achievement was the advancement of a theory he called "immate ...
, who pre-dated Kant, asserted that matter, independent of an observant mind, is metaphysically impossible. Qualities associated with matter, such as shape, color, smell, texture, weight, temperature, and sound are all dependent on minds, which allow only for relative perception, not absolute perception. The complete absence of such minds (and more importantly an omnipotent mind) would render those same qualities unobservable and even unimaginable. Berkeley called this philosophy
immaterialism Subjective idealism, or empirical idealism, is a form of philosophical monism that holds that only minds and mental contents exist. It entails and is generally identified or associated with immaterialism, the doctrine that material things do no ...
. Essentially there could be no such thing as matter without a mind.


Schopenhauer's critique

Schopenhauer Arthur Schopenhauer ( , ; 22 February 1788 – 21 September 1860) was a German philosopher. He is best known for his 1818 work ''The World as Will and Representation'' (expanded in 1844), which characterizes the phenomenal world as the prod ...
claimed that
Kant Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German Philosophy, philosopher and one of the central Age of Enlightenment, Enlightenment thinkers. Born in Königsberg, Kant's comprehensive and systematic works in epistemolo ...
used the word ''noumenon'' incorrectly. He explained in his "
Critique of the Kantian philosophy "Critique of the Kantian philosophy" (German: "Kritik der Kantischen Philosophie") is a criticism Arthur Schopenhauer appended to the first volume of his ''The World as Will and Representation'' (1818). He wanted to show Immanuel Kant's errors so ...
", which first appeared as an appendix to ''
The World as Will and Representation ''The World as Will and Representation'' (''WWR''; german: Die Welt als Wille und Vorstellung, ''WWV''), sometimes translated as ''The World as Will and Idea'', is the central work of the German philosopher Arthur Schopenhauer. The first edition ...
'':
The difference between abstract and intuitive cognition, which Kant entirely overlooks, was the very one that ancient philosophers indicated as φαινόμενα 'phainomena''and νοούμενα 'nooumena'' the opposition and incommensurability between these terms proved very productive in the philosophemes of the
Eleatics The Eleatics were a group of pre-Socratic philosophers in the 5th century BC centered around the ancient Italian Greek colony of Elea ( grc, Ἐλέα), located in present-day Campania in southern Italy. The primary philosophers who are associat ...
, in
Plato Plato ( ; grc-gre, Πλάτων ; 428/427 or 424/423 – 348/347 BC) was a Greek philosopher born in Athens during the Classical period in Ancient Greece. He founded the Platonist school of thought and the Academy, the first institution ...
's doctrine of
Ideas In common usage and in philosophy, ideas are the results of thought. Also in philosophy, ideas can also be mental representational images of some object. Many philosophers have considered ideas to be a fundamental ontological category of being. ...
, in the dialectic of the
Megarics The Megarian school of philosophy, which flourished in the 4th century BC, was founded by Euclides of Megara, one of the pupils of Socrates. Its ethical teachings were derived from Socrates, recognizing a single good, which was apparently combined ...
, and later in the
scholastics Scholasticism was a medieval school of philosophy that employed a critical organic method of philosophical analysis predicated upon the Aristotelian 10 Categories. Christian scholasticism emerged within the monastic schools that translate ...
, in the conflict between
nominalism In metaphysics, nominalism is the view that universals and abstract objects do not actually exist other than being merely names or labels. There are at least two main versions of nominalism. One version denies the existence of universalsthings t ...
and
realism Realism, Realistic, or Realists may refer to: In the arts *Realism (arts), the general attempt to depict subjects truthfully in different forms of the arts Arts movements related to realism include: *Classical Realism *Literary realism, a move ...
. This latter conflict was the late development of a seed already present in the opposed tendencies of Plato and
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of phil ...
. But Kant, who completely and irresponsibly neglected the issue for which the terms φαινομένα and νοούμενα were already in use, then took possession of the terms as if they were stray and ownerless, and used them as designations of things in themselves and their appearances.
The noumenon's original meaning of "that which is thought" is not compatible with the "
thing-in-itself In Kantian philosophy, the thing-in-itself (german: Ding an sich) is the status of objects as they are, independent of representation and observation. The concept of the thing-in-itself was introduced by the German philosopher Immanuel Kant, and ...
," the latter being Kant's term for things as they exist apart from their existence as images in the mind of an observer. In a footnote to this passage, Schopenhauer provides the following passage from the ''Outlines of Pyrrhonism'' (Bk. I, ch. 13) of
Sextus Empiricus Sextus Empiricus ( grc-gre, Σέξτος Ἐμπειρικός, ; ) was a Ancient Greece, Greek Pyrrhonism, Pyrrhonist philosopher and Empiric school physician. His philosophical works are the most complete surviving account of ancient Greek and ...
to demonstrate the original distinction between phenomenon and noumenon according to ancient philosophers: νοούμενα φαινομένοις ἀντετίθη Ἀναξαγόρας ('Anaxagoras opposed what is thought to what appears.')


See also

*
Always already Always already is a philosophical term regarding the perception of phenomena by the mind of an observer. The features of a phenomenon that seem to precede any perception of it are said to be "always already" present. Development "Always already" ...
* Anatta *
Condition of possibility In philosophy, condition of possibility (german: Bedingungen der Möglichkeit) is a concept made popular by the German philosopher Immanuel Kant, and is an important part of his philosophy. A condition of possibility is a necessary framework fo ...
* Essence–energies distinction *
Haecceity Haecceity (; from the Latin ''haecceitas'', which translates as "thisness") is a term from medieval scholastic philosophy, first coined by followers of Duns Scotus to denote a concept that he seems to have originated: the irreducible determination ...
*
Hypokeimenon ''Hypokeimenon'' (Greek: ὑποκείμενον), later often material substratum, is a term in metaphysics which literally means the "underlying thing" (Latin: ''subiectum''). To search for the ''hypokeimenon'' is to search for that substance t ...
*
Ineffability Ineffability is the quality of something that surpasses the capacity of language to express it, often being in the form of a taboo or incomprehensible term. This property is commonly associated with philosophy, aspects of existence, and similar ...
*
Master argument :''See Diodorus Cronus § Master argument for the classical master argument related to the problem of future contingents.'' The master argument is George Berkeley's argument that mind-independent objects do not exist because it is impossible to ...
by
George Berkeley George Berkeley (; 12 March 168514 January 1753) – known as Bishop Berkeley (Bishop of Cloyne of the Anglican Church of Ireland) – was an Anglo-Irish philosopher whose primary achievement was the advancement of a theory he called "immate ...
*
Observation Observation is the active acquisition of information from a primary source. In living beings, observation employs the senses. In science, observation can also involve the perception and recording of data via the use of scientific instruments. The ...
*
Qualia In philosophy of mind, qualia ( or ; singular form: quale) are defined as individual instances of subjective, conscious experience. The term ''qualia'' derives from the Latin neuter plural form (''qualia'') of the Latin adjective '' quālis'' () ...
*
Schopenhauer's criticism of the Kantian philosophy "Critique of the Kantian philosophy" (German: "Kritik der Kantischen Philosophie") is a criticism Arthur Schopenhauer appended to the first volume of his ''The World as Will and Representation'' (1818). He wanted to show Immanuel Kant's errors s ...
*
Transcendental idealism Transcendental idealism is a philosophical system founded by German philosopher Immanuel Kant in the 18th century. Kant's epistemological program is found throughout his '' Critique of Pure Reason'' (1781). By ''transcendental'' (a term that des ...
*
Unobservable An unobservable (also called impalpable) is an entity whose existence, nature, properties, qualities or relations are not directly observable by humans. In philosophy of science, typical examples of "unobservables" are the force of gravity, causat ...


Notes


References


Bibliography

*


External links


''The surd of metaphysics; an inquiry into the question: Are there things-in-themselves?'' (1903)
by
Paul Carus Paul Carus (; 18 July 1852 – 11 February 1919) was a German-American author, editor, a student of comparative religion
, 1852–1919 {{Authority control Abstract object theory Abstraction Cognition Concepts in aesthetics Concepts in epistemology Concepts in metaphilosophy Concepts in metaphysics Concepts in the philosophy of mind Concepts in the philosophy of science Enlightenment philosophy Epistemological theories Epistemology German idealism History of philosophy Idealism Immanuel Kant Intellectual history Kantianism Knowledge Mental processes Metaphysical theories Metaphysics Metaphysics of mind Ontology Perception Philosophical arguments Philosophical concepts Philosophical problems Philosophical theories Platonism Possibility Pyrrhonism Qualia Reality Senses Thought