Branches of philosophy

Figure 1. The branches of philosophy (red) and their links with allied sciences (yellow).

Plato, in the Republic, compares those who live by conventional wisdom and who are dominated by their senses and appetites to prisoners living in an cave, chained since childhood, so that they can see only what is directly in front of them.

Figure 2. Plato, in the Republic, compares those who live by conventional wisdom and who are dominated by their senses and appetites to prisoners living in an cave, chained since childhood, so that they can see only what is directly in front of them. A fire behind them throws the shadows of objects on to the wall. Our knowledge of real subjects is as incomplete as those prisoners' would be. It is only through philosophy and intelligent reflection that we can escape from this world of shadows of puppets and see true realities. If anyone returned to the cave to point out the illusion, he would be ridiculed.

Plato's One Over Many Argument applies to every general word, whether material such as tree or abstract such as piety.

Figure 3. Plato's One Over Many Argument applies to every general word, whether material such as tree or abstract such as piety. "Tree" may describe a number of different examples. These change and none of them is perfect, but still we recognize the general class. The Form or Idea is eternal, unchanging, incorruptible, and immaterial. It is somehow simultaneously both more real and more ideal than any particular manifestation. These perfect Forms can never be attained in the everyday world and can only be known by the intellect, not the senses. True knowledge is therefore the knowledge of Forms. Plato is not clear as to whether particular examples are caused by the Forms or are merely a shadowy likeness.

Aristotle's Third Man argument

Figure 4. Aristotle advanced several commonsense criticisms of the Theory of Forms. Plato had given the fact that things have common characteristics much too great significance. It was unnecessary to postulate a separate mystical realm where pure being exists and which is never experienced. Forms, he contended, are no more than those qualities that are experienced as similar within things. Aristotle also put forward the well-known Third Man argument. Wherever two entities are discovered with a common quality, Plato postulates a Form (A). But the original entity and the Form now also share a common quality. Therefore we have to postulate yet another Form in which they both share (B). This process can be extended infinitely.

The problem of universals, or words like 'man' that apply to many examples, has three differing explanations.

Figure 5. The problem of universals, or words like "man" that apply to many examples, has three differing explanations. For the Realists, like Plato, universals are entities called Forms or Ideas (A) that exist independently of the instances of these. The universal idea of man is more real than the particular men who exist in the world. For Conceptualists (B) universals are purely concepts in the minds of men, so the universal idea of man comes after and is based on particular men. The Nominalist (C) holds that every concept is a specific individual concept — there is nothing in its nature that makes it general.

Zeno of Elea (fl. 450 BC) questioned our notions of time and change through a paradox.

Figure 6. Zeno of Elea (fl. 450 BC) questioned our notions of time and change through a paradox. Achilles can run ten times as fast as the tortoise, who has a ten-unit start. When Achilles has run his first ten the tortoise will be one ahead. When Achilles has run that one the tortoise will be one-tenth ahead, and so on. Logically, Achilles cannot win, because Zeno defines the race mathematically, but our senses tell us that Achilles does win.

Avicenna (980–1037) (A) and Averrhoes (1126–1198) (B) were Islamic philosophers.

Figure 7. Avicenna (980–1037) (A) and Averrhoes (1126–1198) (B) were Islamic philosophers. Two centuries before Aquinas, Avicenna attempted an Islamic scholasticism, a synthesis of the best of ancient Greek philosophy with the teachings of Mohammed. Some of his work was attacked as heretical, but later both the Jew Maimonides and the Christian Aquinas adopted Avicenna's suggestion that in God's essence and existence are one. Averrhoes, born in Moorish Spain, became best known in Christendom for commentaries on Aristotle. Translations of these were at one time regularly bound up with Latin versions of Aristotle's Works. Averrhoes founded a Muslim philosophy of religion.

Socrates frequented the marketplace in Athens, spending his time, to the annoyance of his wife, arguing and challenging the conventional wisdom of the day rather than earning a living.

Figure 8. Socrates frequented the marketplace in Athens, spending his time, to the annoyance of his wife, arguing and challenging the conventional wisdom of the day rather than earning a living. Few could stand up to his style of cross-examination and he made many enemies. Eventually he was tried on charges of corrupting youth and being irreligious and was sentenced to death. He could have escaped but argued that it would be inconsistent with all that he had taught. He drank hemlock and died comforting his friends.

Philosophy (from philosophos, lover of wisdom) is any body of doctrine or opinion as to the nature and ultimate significance of human experience considered as a whole. It is perhaps more properly applied to the critical evaluation of all claims to knowledge – including its own and anything that is presupposed about its own nature and task. In this latter respect, it is widely argued, philosophy differs fundamentally from all other disciplines. What philosophy "is" (what methods philosophers should employ, what criteria they should appeal to, and what goals they should set themselves) is as perennial a question for the philosopher as any other.


Branches of philosophy

Traditionally, philosophers have concerned themselves with four main topic areas: logic, the study of the formal structure of valid arguments; metaphysics, usually identified with ontology – the study of the nature of "Being" or ultimate reality; epistemology, or theory of knowledge, sometimes treated as a branch of metaphysics, and axiology, or theory of value – including aesthetics, the philosophy of taste (especially as applied to the arts), ethics, or moral philosophy, and political philosophy or political science. In modern times as traditional philosophy has yielded up the subject matters of the natural sciences, of other descriptive studies such as psychology and sociology, and of such formal studies as logic and mathematics, all once numbered among its legitimate concerns, philosophers have become increasingly conscious of their critical role. Most now tend to interest themselves in special philosophies, e.g., the philosophy of logic, the philosophy of science, and the philosophy of religion (Figure 1).


Historical development of philosophy

The first attempts to answer distinctively philosophical questions were made from about 600 BC by certain Greek philosophers known collectively as the Presocratics (see pre-Socratic philosophy); their intellectual heirs were Socrates, Plato, and Aristotle, the three towering figures in ancient philosophy. Later ancient philosophies included epicureanism, stoicism, and neoplatonism. Foremost among medieval philosophers were St. Augustine and Thomas Aquinas, both leading churchmen.


Heraclitus flourished about 500 BC in Ephesus.
Heraclitus flourished about 500 BC in Ephesus. As with all other Greek pre-Socratic philosophers, any knowledge of his ideas has to be derived from a few surviving fragments and the often malicious gossip of rivals. His theory that all things are in a state of flux or change stimulated Plato into producing his theory of unchanging ideas that provided stable standards for conduct.


Modern philosophy begins with René Descartes and a parallel development of rationalism and empiricism culminating in the philosophy of Immanuel Kant. The idealism of Georg Hegel and the positivism of Auguste Comte were major forces in the 19th-century philosophy. The dialectical materialism of Karl Marx had its roots in both. The philosophical orientations of most twentieth-century philosophers were developments of Marxism, neokantianism, logical positivism, pragmatism, phenomenology, and existentialism.


The gap between appearance and reality

The search for universal definitions is triggered by the need to discover what things, if any, we can know with absolute certainty. Generally people are aware, at least from time to time, that there is a gap between appearance and reality even if it is only that the person we see in the mirror parts his hair on the right whereas in reality he parts it on the left. In a world where one of the most salient characteristics is change or flux, how do we gain knowledge of the "real"?


Plato, a pupil of Socrates, identified flux with appearances or what we "know" through our senses, appetites and emotions. He argued that reality is something other than that which is perceived by our senses – the so-called Theory of Forms or One Over Many Argument.


Plato believed that what is common to things described by the same name, such as trees, is their Form or Idea (Figure 3). The Form represents being, the particular examples of that form represent becoming, and these two realms are separate. Plato's arguments are illustrated by curiously haunting imagery. His four ultimate categories of existence (and therefore of knowledge) constitute a hierarchy. These levels correspond to the four sections of what Plato called The Line, which he divided first into two unequal parts with each then subdivided in the same proportions. At the top are Ideas and the knowledge of them; immediately underneath, the purest of pure mathematics. For Plato this ideal world is alone truly real. A long way below falls our everyday world: physical objects on top and, under these, shadows and reflections. To move from lower to higher is to pass from shadow to substance.


Yet any such move, up or down, is disturbing. To be confined to the nether region is to be like a prisoner in a cave (Figure 2), seeing nothing but shadows cast by artificial light. For the released prisoner it hurts at first to look at things in the sunlight, and still more to confront the sun itself.


Philosophers were described by Plato as being split between two great armies: those of the Gods and the Giants.
Philosophers were described by Plato as being split between two great armies: those of the Gods and the Giants. The Gods, fighting from the heaven of the ideal world, maintain that ideas are fundamental and are all that has any existence or reality; they are Idealists. The Giants, by contrast, struggle to pull everything down to earth and maintain that matter is primary, or even that it is all there is; they are Materialists. In his dialogue the Sophist. Plato goes on to say that in the great battle between Materialists and Idealists neither side can defend it-self. If, as the Materialists say, reality is what we can grasp with our hands we deny "justice" or "wisdom". If we say only ideas are real, we deny living things.


Schools of philosophy


common sense school

The common sense school was a group of Scottish thinkers, include Thomas Reid and Douglas Stewart, who, reacting against the idealism of George Berkeley and the skepticism of David Hume, affirmed that the truths apparent to the common man – the existence of material objects, the reality of causality, and so on – were genuine, reliable, and not to be questioned.



Conceptualism is a modern term describing a position in scholastic philosophy with respect to the status of universals (general concepts such as chair-ness) that was intermediate between the extremes of nominalism and realism. To a conceptualist, universals indeed exist, but only as concepts common to all human minds and not as things in the world particular objects (such as chairs).



Deism is a theological movement, which gathered momentum in the eighteenth century and which rejects continuing divine involvement in the details of the world. For the deist, God's action consists of setting the universe in operation at the beginning of time.



Determinism is the philosophical theory that all events are determined (inescapably caused) by preexisting events which, when considered in the context of inviolable physical laws, completely account for the subsequent events. The case for determinism has been variously argued from the inviolability of the laws of nature and from the omniscience and omnipotence of God. Determinism is often taken to be opposed to the principles of free will and indeterminacy.



Dualism is any religious or philosophical system characterized by a fundamental opposition of two independent or complimentary principles. Among religious dualisms are the unending conflict of good and evil spirits envisaged in Zoroastrianism and the opposition of light and darkness in Jewish apocalyptic, Gnosticism, and Manichaeism. The Chinese complimentary principles of ying and yang exemplify a cosmological dualism while the mind-body dualism of Descartes is the best-known philosophical type. Dualism is often opposed to monism and pluralism.



In philosophy, empiricism is the view that knowledge can be derived only from sense experience. Modern empiricism, fundamentally opposed to the rationalism that derived knowledge by deduction from principles known as a priori, was developed in the philosophies of Locke, Berkeley, and Hume. Other thinkers in the "British empiricist tradition" include John Stuart Mill and the Americans John Dewey and William James.



Idealism is a name adopted by several schools of philosophy, all of which in some way assert the primacy of ideas, either as the sole authentic stuff of reality or as the only medium through which we can have knowledge or experience of the world. Idealisms are commonly contrasted both with the various types of realism and with philosophical materialism. They are often associated with methodological rationalism because they usually seem to owe more in reasoning upon a priori principles than to any appeal to experience. The idealism of Plato, in which ideas were held to have an external objectivity, is unrepresentative of modern varieties, of which that of George Berkeley is archetypal. Kant and Hegel were foremost in the German idealist tradition, while T. H. Green, F. H. Bradley, and J. Royce were representative of more recent-speaking idealists. Idealism has, however, been in eclipse since the start of the twentieth century.



Instrumentalism, also known as experimentalism, is a term in philosophy describing the development of pragmatism promoted by John Dewey. It is based on the contention that ideas are validated solely by their usefulness in solving problems.


logical positivism

Logical positivism incorporates the doctrines of the 'Vienna Circle,' a group of philosophers founded by the German philosopher Moritz Schlick (1882–1936) and which included Rudolf Carnap. At the heart of logical positivism was the assertion that apparently factual statements that were not sanctioned by logical or mathematical convention were meaningful only if they could conceivably be empirically verified. Thus only mathematics, logic, and science were deemed meaningful; ethics, metaphysics, and religion were considered worthless. The influence of logical positivism evaporated after World War II.



Materialism is the position in philosophy, standing in opposition to idealism, which asserts the ontologic primacy of matter. In psychology, materialism is any theory denying the existence of mind and regarding mental phenomena to be the mere outworking of purely physico-mechanical processes in the brain. In the philosophy of religion, materialism is any synthesis denying the existence of an immortal human soul. The earliest thoroughgoing materialists were the classical Atomists (see atomism), in particular Democritus and Lucretius. The growth of modern science brought a revival of materialism, which many have argued is a prerequisite for scientific thought, particularly in the field of psychology. Other philosophers, however, have argued against this view, recognizing the arbitrariness of the materialist hypothesis.



Monism is any philosophical system asserting the essential unity of things, i.e., that all things belong to one category – material (see materialism), mental (see idealism), spiritual (pantheism), or some other essence. Monism is contrasted with the various kinds of dualism.



Phenomenology is a school of philosophy based on a method of approach due to Edmund Husserl (1859–1938). Unlike the naturalist, who describes objects without reference to the subjectivity of the observer, the phenomenologist attempts to describe the "invariant essences" of objects as objects "intended" by consciousness. As a first step toward achieving this, he or she performs the "phenomenological reduction", which involves as far as possible a suspension of all preconceptions about experience.



Positivism is a philosophical theory of knowledge associated with the 19th-century French philosopher Auguste Comte. It holds that the observable, or "positive," data of sense experience consititute the sole basis for assertions about matters of fact; only the truths of logic and mathematics are additionally admitted. The speculative claims of theology and metaphysics, regarded as the primitive antecedents of "positive" or scientific thought, are discounted.



Pragmatism is a philosophical theory of knowledge whose criterion of truth is relative to events and not, as in traditional philosophy, absolute and independent of human experience. A theory is pragmatically true if it "works" – if it has an intended or predicted effect. All human undertakings are viewed as attempts to solve problems in the world of action; if theories are not trial solutions capable of being tested, they are pointless. The philosophy of pragmatism was developed in reaction to late nineteenth-century idealism mainly by the American philosophers Charles Peirce, John Dewey, and William James.


pre-Socratic philosophy

'Pre-Socratic philosophy' is a term applied to the thought of the early Greek philosophers (c. 600–400 BC) whose work came before the influence of Socrates. Their works survive mostly in obscure fragments, but their fame and importance lie in their being the first to attempt rational explanations of the universe. They are grouped into the Ionian school in Asia Minor (Thales, Anaximander, Anaximenes, Xenophanes, Heraclitus, Anaxagoras, and the Atomists, Leucippus and Democritus) and Pythagoras and the Eleatics Parmenides, Zeno, and Empedocles in southern Italy and Sicily. Protagoras and the pre-Sophists are usually also included.


process philosophy

Process philosophy is a radical alternative to positivistic philosophies, in which the physicists' 'spatial' notion of time is rejected and interest centers in the developmental process. First propounded by Henri Bergson, process philosophy influenced William James, George Santayana, and Alfred North Whitehead.



Rationalism is a philosophical approach based on the view that reality has a logical structure accessible to deductive reasoning and proof, and holding, as against empiricism, that reason unsupported by sense experience is a source of "synthetic knowledge" – knowledge, primarily of certain fundamental concepts and principles in logic and mathematics, which, it is argued, cannot be denied without contradiction and yet cannot be dismissed as merely analytic. Major rationalists in modern philosophy include Descartes, Spinoza, and Leibniz.



In philosophy, realism is a term with two main technical uses. Philosophers who believe, as Plato did, that universals exist in their own right, and so independently of perceived objects, are traditionally labeled "realists." Realism in this sense is opposed to nominalism. On the other hand, realism also describes the view that perceived objects exist independently of our perceptions of them. Realism in this sense is opposed to the extreme empiricism of such as Berkeley and Hume.



Reductionism is the idea that nature can be understood by taking it apart. In other words, knowing the lowest-level details of how things work (at, say, the level of subatomic physics) reveals how higher-level phenomena come about. This is a bottom-up way of looking at the universe, and is the exact opposite of holism.



Skepticism is a philosophical attitude of doubting all claims to knowledge, chiefly on the ground that the adequacy of any proposed criterion is itself questionable. Examples of thoroughgoing skeptics, wary of dogmatism in whatever guise, were Pyrrho of Elis ("Pyrrhonism" and "skepticism" are virtual synonyms) and David Hume. Other thinkers, among them Augustine, Erasmus, Montaigne, Pascal, Bayle, and Kierkergaard, sought to defend faith and religion by directing skeptical arguments against the epistemological claims of rationalism and empiricism. Pragmatism and Kant's critical philosophy represent two influential attempts to resolve a skeptical dilemma.



Solipsism is an extreme form of subjective idealism based on an argument to the effect that since I can apprehend nothing that is not part of my experience, there can be no legitimate grounds for affirming the existence of an external world that is independent of my experience of it.



Teleology (from the Greek telos meaning "end") is the doctrine that phenomena occur for a purpose. Aristotle argued that to have a complete understanding of anything its "final cause," its purpose in existing, had to be taken into account. The teleological argument for God's existence is that from design.


It was often used, from the seventeenth century on, as a theological argument in favor of pluralism, the argument being that God would not have gone to the trouble of creating uninhabited worlds. Among those to give prominence to teleological reasoning were Jacques Henri Bernadin de Saint-Pierre and John Locke. Opponents of it included Ludwig Büchner.