Philosophy (literally 'love of wisdom', from the Ancient Greek word φιλοσοφία (philosophía), which comes from φίλος (phílos) and σοφία (sophía), meaning friend/lover and wisdom respectively) is an academic discipline concerned with the most fundamental and general concepts and principles involved in thought, action, and reality. Philosophy progresses according to various methods of rational inquiry.
Philosophy is accurately described as both the first science and the "Queen of the Sciences". Until the late 19th century, what is now called "science" was subsumed under Natural Philosophy.
Philosophy in the West has its origins in Ancient Greece, ca. 600 B.C. After the fall of the Roman Empire, much of Greek philosophy was lost to the West, preserved only in the Arab world until the time of the Crusades and the Moorish conquest of Spain. Contact with Arab philosophers (especially al-Ghazali, who developed a version of the Cosmological argument and wrote more than 70 books, ibn Rushd, and ibn Sina) kick-started the largely dormant philosophical tradition in Europe, beginning the "Academic period", which ended with Descartes and the beginning of the Enlightenment. The late 19th and early 20th centuries saw a major split between Analytic philosophy (or Anglophone philosophy) and Continental philosophy. This divide can be traced back to the late 19th century and a split in focus between Gottlob Frege and the intellectual descendants of Friedrich Hegel.
The main branches of philosophy are Metaphysics, which is (broadly speaking) the study of what-is; Ethics, the study of correct action; Logic, the rules (both formal and informal) of reason; Epistemology, the study of knowledge, and Aesthetics, the study of the nature of beauty and the artistic criteria of judgment. Notable sub-branches include Philosophy of Science, Philosophy of language, Philosophy of Religion, Political Philosophy, and Philosophy of mind.
Branches of Analytic Philosophy
Broadly speaking, there are a number of topics one would expect to fall under the title Philosophy:
- Logic: The analysis of terms, propositions and the principles of reasoning
- Metaphysics: The analysis of concepts which transcend physical science, including the Philosophy of mind.
- Epistemology: The analysis of the nature of knowledge, how we know, and what we can and cannot know.
- Ethics: The analysis of the nature of morality and morals, how and why we determine right from wrong.
- Political Philosophy: The analysis of the nature of the human public sphere, which one may consider the ethics of the way society is arranged.
- Philosophy of Science: The analysis of scientific concepts and methodology, which concerns itself mostly with the foundations of science, and interdisciplinary areas.
- Aesthetics: The analysis of the nature and experience of art and beauty.
History of Philosophy
Conventionally the History of Western Philosophy is divided into four eras: Ancient, Medieval, Modern and Contemporary.
The Ancient era starts with the Presocratic philosophers and goes until the fall of the Roman empire; The Medieval goes until the end of the Middle Ages; The Modern up to the 20th century, and The Contemporary up to present.
Philosophy in the West begins with Thales of Miletus, who was the first astronomer in recorded history to accurately predict a solar eclipse. The Milesian School founded by Thales included Anaxagoras, Anaximander, and Anaximenes. It was Pythagoras (ca. 582 - 504 BC) who first brought Philosophy into connection with practical life; he also gave Philosophy its name "the love of wisdom". Closely related to Milesianis is the work of Heraclitus of Ephesus. Around the time of Heraclitus, Parmenides of Elea, with his pupil Zeno, raised some serious objections to the project of Milesian philosophy. These objections laid the groundwork for Socrates and his pupil Plato, and Aristotle, as well as the Atomists, Democritus and Leucippus. After the Greek philosophical golden age other systems appeared, like Cynicism, Stoicism, Epicureanism and Skepticism. At the closing period of Greek philosophy Neoplatonism was founded by Plotinus of Lycopolis. Parmenides' thoughts influenced Plato and through him, all western philosophy. While Plato's dialogues are among the supreme philosophical works of the western tradition, it was Parmenides who established the implicit framework of their debates. 
Saint Augustine (354-430 A.D.), carrying some traces of Plato, has been of momentous importance in the development of Christian thought.
It was Plato, who made idea a technical term in philosophy, suggesting a world of abstract ideas idealism; Aristotle on the contrary, shows the real that dwells in the objects of sense; he thought that the natural world needed to be studied and observed for knowledge to be gained. His theory of act and potentiality, of form and matter, is a new solution of the relations between the permanent and the changing; following the first attempts of the Presocratic Philosophy that conducted to Materialism, Aristotle reproaches the Ionian philosophers in general with attempting to explain the evolution of the world without the Nous (intelligence); he regarded Protagoras, who first introduced a spiritual principle, as a sober man among the inebriated. 
Since that age up to present days, idealism and materialism have been the two main tendencies in the field of philosophy.
In the West, until the twelfth century, little was known of Plato and Aristotle, except a few dialogues and some treatises on logic. St. Anselm (1O33-1109) made a first attempt at systematizing Scholastic philosophy. Some decades later, the Arabic and Byzantine thinkers entered into relation with Western culture, and affecting a philosophical revival; Al-Ghazali, founder of the Ash'ari school of Atomism, entered Europe through ibn Rushd; translations of the ancient authors were made and the philosophical works of ibn Sina and ibn Rushd became better known. In the thirteenth century important names appeared like: Maimonides, Alexander of Hales (ca. 1185 - 1245), St. Bonaventure, Albertus Magnus, Roger Bacon, St. Thomas Aquinas, and in the following century: William of Occam, Eckhart and Nicholas of Cusa, and later, Thomas More and Grotius. (cf: Ibidem Catholic Encyclopedia.)
The main figures from this era are: Thomas Hobbes, Rene Descartes , Gottfried Leibniz, David Hume, Immanuel Kant, Arthur Schopenhauer, Auguste Comte, John Stuart Mill, Herbert Spencer, Karl Marx and Friedrich Nietzsche.
The most widely accepted scheme, that which still governs the division of the branches of philosophy is due to Christian Wolff (1679-1755):
This scheme is as follows:
1. Logic. 2. Speculative Philosophy.
- Ontology, or General Metaphysics.
- Special Metaphysics.
- o Theodicy (the study of God).
- o Cosmology (the study of the World).
- o Psychology (the study of Man).
3. Practical Philosophy.
- All the interests of my reason, speculative as well as practical, combine in the three following questions:
- 1. What can I know? 2. What ought I to do? 3. What may I hope? Immanuel Kant.
The Twentieth Century
This century has offered a wide diversity of orientations. The best known are the French school of Existentialism, the German study of Phenomenology, the Positivists of the Vienna Circle, and the post-Positivist Analytic movement.
The earliest existentialist was the Christian philosopher Søren Kierkegaard. Other prominent existentialists include Henri Bergson (1859 – 1941), winner of the 1927 Nobel Prize in Literature, Karl Jaspers, and the noted philosopher, playwright, and novelist Jean-Paul Sartre (1905 – 1980).
Phenomenology and Hermeneutics
Edmund Husserl (1859 – 1938) founder of phenomenology, Martin Heidegger (1889 – 1976), and Hans-Georg Gadamer (1900-2002) are the core writers here.
The Vienna Circle philosophers, most notably Moritz Schlick, Rudolf Carnap, Herbert Feigl, the famous mathematician Kurt Godel, and Otto Neurath established a rigorous tradition called "Logical Empiricism", but usually referred to as logical positivism. This tradition, a stricter development of Auguste Comte's positivism, is no longer an active research program, unlike Existentialism and Phenomenology.
Often thought of as a logical positivist Karl Popper was in fact a fierce critic.
Structuralism is a theory that uses culturally interconnected signs to reconstruct systems of relationships rather than studying isolated, material things in themselves, with proponents such as Swiss linguist Ferdinand de Saussure (1857 – 1913), Russian linguist Roman Jakobson (1896 – 1982), French anthropologist Claude Lévi-Strauss (1908 – 2009 ), French literary critic Roland Barthes (1915 – 1880), and French philosopher and historian Michel Foucault (1926 – 1984). Foucault, who is often classified as a post-structuralist, is renowned for historical studies that reveal the sometimes morally disturbing power relations inherent in social practices. 
The Frankfurt School , a group of researchers associated with the Institut für Sozialforschung (Institute of Social Research), founded in 1923 as an autonomous division of the University of Frankfurt; members were Max Horkheimer, Theodor Wiesengrund Adorno, his friend Walter Benjamin (1892 – 1940), a German essayist and critic known for his synthesis of eccentric Marxist theory and Jewish messianism and Herbert Marcuse (1898 – 1979). Jürgen Habermas (1929-), a critic of Positivism is the most famous German professor of the tradition of critical theory.
British philosophers from the beginning of the 20. Century are: George Edward Moore (1873 – 1958), Bertrand Russell (1872 – 1970), philosopher, mathematician, and social reformer, and the Austrian Ludwig Wittgenstein (1889 – 1951), who taught in Cambridge.
Some scholars have said that American philosophers’ focus on the interconnections of theory and practice, on experience and community, but different concerns and themes have waxed or waned at different times. Through the middle of the 20th century, at least, American philosophers were actively engaged in shaping and reflecting the development of American culture.
Founded by Ralph Waldo Emerson (1803 - 1882) who became the most widely known man of letters in America in the 19th century. Transcendentalism, of which Emerson was the leading figure, resembled British Romanticism in its precept that a fundamental continuity exists between man, nature, and God, or the divine... Matter and spirit are not opposed. 
Philosophy of Religion
The best-known American philosopher of religion is without a doubt Alvin Plantinga (1932- ) at the University of Notre Dame; also well-known are John Hick (1922- ), who, though born in England, did much of his work at Claremont University in southern California, and professor William Alston (1921- ) working at Syracuse University.
America has produced a great many philosophers in the Analytic tradition. A brief list of notables:
- W.V.O. Quine
- Donald Davidson
- Saul Kripke
- John Searle
- Paul Grice
- Richard Montague
- Hilary Putnam
- Nelson Goodman
- Kit Fine
- Ted Sider
- Robert Stalnaker
- Daniel Dennett
Especially well-known to non-philosophers is John Rawls (1921 - 2002) (political and moral philosophy) author of A Theory of Justice (1971), Political Liberalism (1993), The Law of Peoples (1999), and Justice as Fairness: A Restatement (2001).
- History of Philosophy
- History of Philosophy - Resources
- A History of American Philosophy Book by Herbert W. Schneider.
- Edited versions of important texts in philosophy.