Philosophy has a long history. Generally, philosophers divide the history of Western philosophy into ancient philosophy, medieval philosophy, modern philosophy, and contemporary philosophy.
Western Philosophy is generally said to begin in the Greek cities of western Asia Minor (Ionia) with Thales of Miletus, who was active around 585 B.C. and left us the opaque dictum, "All is water." His most noted students were Anaximenes of Miletus and Anaximander ("All is air").
Other thinkers and schools appeared throughout Greece over the next couple of centuries. Among the most important were:
Heraclitus, who stressed the transitory and chaotic nature of all things ("All is fire"; "We cannot step into the same river twice").
Anaxagoras, who conversely asserted that reality was so ordered that it must be in all respects governed by Mind.
The Pluralists and Atomists (Empedocles, Democritus) who tried to understand the world as composite of innumerable interacting parts; and the Eleatics Parmenides and Zeno who both insisted that All is One and change is impossible. Parmenides and his school emphasized the numerical, mathematical character of the world and of truth.
The Sophists, traveling professional teachers of varied philosophical affinity, became known (perhaps unjustly) for claiming that truth was no more than opinion and for teaching people to argue fallaciously to prove whatever conclusions they wished.
This whole movement gradually became more concentrated in Athens, which had become the dominant city-state in Greece.
There is considerable discussion about why Athenian culture encouraged philosophy, but one popular theory says that it occurred because Athens had a direct democracy. It's known from Plato's writings that many sophists maintained schools of debate, were respected members of society, and well paid by their students. It's also well known that orators had tremendous influence on Athenian history, possibly even causing its failure (See Battle of Miletus). The theory fills in the blanks by saying that the Sophists' students wanted to acquire the skills of an orator in order to influence the Athenian Assembly, and thereby grow wealthy and respected. Since winning debates led to wealth, the subjects and methods of debate became highly developed. Note that Western and American culture maintain this trait. Culturally, Westerners are very Greek.
The key figure in transforming Greek philosophy into a unified and continuous project - the one still being pursued today - is Socrates, who studied under several Sophists and then spent much of his life, we are told, engaging everyone in Athens in discussion trying to determine whether anyone had a very good idea what they were talking about, especially when they talked about important matters like justice, beauty and truth. He wrote nothing, but inspired many disciples. In his old age he became the focus of the hostility of many in the city who saw philosophy and sophistry, interchangeably, as destroying the piety and moral fibre of the city; he was executed in 399 B.C.
His most important student was Plato, who wrote a number of philosophical dialogues using his master's methods of inquiry to examine problems. The early dialogues demonstrate something like Socrates' own fairly inconclusive style of inquiry. The "middle" ones develop a substantive metaphysical and ethical system to resolve these problems. Central ideas are the Theory of Forms, that the mind is imbued with an innate capacity to understand and apply concepts to the world, and that these concepts are in a significant way more real, or more basically real, than the things of the world around us; the immortality of the soul, and the idea that it too is more important than the body; the idea that evil is a kind of ignorance, that only knowledge can lead to virtue, that art should be subordinate to moral purposes, and that society should be ruled by a class of philosopher kings. In the later dialogues Socrates figures less prominently, and the Theory of Forms is cast in doubt; more directly ethical questions become the focus.
Plato founded the Academy of Athens, and his most outstanding student there was Aristotle. Possibly Aristotle's most important and long-lasting work was his formalization of logic. It appears that Aristotle was the first philosopher to categorize every valid syllogism. A syllogism is a form of argument that is guaranteed to be accepted, because it is known (by all educated persons) to be valid. A crucial assumption in Aristotelian logic is that it has to be about real objects. Two of Aristotle's syllogisms are invalid to modern eyes. For example, "All A are B. All A are C. Therefore, some B are C." This syllogism fails if set A is empty.
Medieval philosophy was greatly concerned with the nature of God, and the application of Aristotle's logic and thought to every area of life.
If God exists at all, surely He is the most important feature of the universe, and therefore worthy of study. One continuing interest in this time was to prove the existence of God, through logic alone, if possible.
One early effort was the Cosmological Argument, conventionally attributed to Thomas Aquinas. The argument roughly, is that everything that exists has a cause. Therefore, there must be an uncaused first cause, and this is God. Aquinas also adapted this argument to prove the goodness of God. Everything has some goodness, and the cause of each thing is better than the thing caused. Therefore, the first cause is the best possible thing. Similar arguments are used to prove God's power and uniqueness.
Another important argument proof of the existence of God was the Ontological Argument. Basically, it says that God has all possible good features. Existence is good, and therefore God has it, and therefore exists.
The application of Aristotelian logic proceeded by having the student memorize a rather large set of syllogisms. The memorization proceeded from diagrams, or learning a key sentence, with the first letter of each word reminding the student of the names of the syllogisms.
Each syllogism had a name, for example "Modus Ponens" had the form of "If A is true, then B is true. A is true, therefore B is true."
Most university students of logic memorized Aristotle's 19 syllogisms of two subjects, permitting them to validly connect a subject and object. A few geniuses developed systems with three subjects, or described a way of elaborating the rules of three subjects.
As well as Aquinas, other important names from the medieval period include Duns Scotus and Peter Abelard.
Modern philosophy generally means philosophy from 1600 until about 1900, and includes many distinguished early modern philosophers, such as Rene Descartes, John Locke, David Hume, and Immanuel Kant. Nineteenth-century philosophy is often treated as its own period, as it was dominated by post-Kantian German and idealist philosophers like Georg Hegel, Karl Marx, and F. H. Bradley; other important thinkers were John Stuart Mill, Arthur Schopenhauer and Friedrich Nietzsche.
For much of the twentieth century, philosophy ran along two fairly independent - and not infrequently antagonistic - streams, roughly corresponding with whether the philosopher in question belonged to the English-speaking world - the British Isles, North America, Australasia - or continental Europe. The former approaches, which began with mathematical logic, continued through logical positivism and later linguistic philosophy and ordinary language philosophy, were broadly dubbed "analytic philosophy," interchangeably with "Anglo-American philosophy." The latter, which initially consisted mainly in phenomenology and existentialism, and later came to incorporate a great deal of Marxist and psychoanalytic social theory, literary criticism, and structuralism and post-structuralism, was dubbed "continental philosophy." By the end of the twentieth century, the two streams freely, if still not frequently, interacted, and an increasing number of professional philosophers were of the opinion that the "analytic/continental" distinction at least did not determine the "good philosophy/ bad philosophy" distinction, and arguably didn't pick out any terribly useful distinction at all.
Analytic philosophers, including Bertrand Russell, G. E. Moore, and Ludwig Wittgenstein, were centered in Oxford and Cambridge, and were joined by logical empiricists emigrating from Austria and Germany (for example, Rudolf Carnap) and their students and others in the United States (such as, W. V. Quine, Donald Davidson, and Saul Kripke, and other English-speaking countries (for example, A. J. Ayer). Gottlob Frege, a German who never worked in the English-speaking world, is arguably the foundation of this tradition, but it began with Russell and Moore in Cambridge at the turn of the century. Russell, A.N. Whitehead, and Wittgenstein (an Austrian) did groundbreaking philosophical work in math and logic. This quickly connected them with the Logical Positivists, a group of scientists and philosophers in Vienna centred around Carnap, Otto Neurath, and Moritz Schlick, and with the logical empricists in Berlin, centred around Reichenbach and Hempel, and later with a number of brilliant schools of logicians that sprang up in Poland.
During the thirties members of these various groups migrated to the United States, helping to lay the grounds for American analytic philosophy. W.V. Quine , who was influenced by all of these (particularly Carnap) is perhaps the key figure here. Also during the thirties Ludwig Wittgenstein came to doubt the philosophical tenability of the very elaborately logic-based philosophy he had earlier done, and stressed the importance of studying ordinary language and practical usage, as being crucial to untangling philosophy. His work was initially influential at Oxford, and after the posthumous publication of his many manuscripts, has spread through all of philosophy.
On the continent of Europe (especially Germany and France), the phenomenologist Germans Edmund Husserl and Martin Heidegger led the way, followed soon by Jean-Paul Sartre and other existentialists; this led via other "isms" to postmodernism, which dominates schools of critical theory as well as philosophy departments in France and Germany, which continue the projects that these philosophers have pursued.
See also: List of philosophers for another list of philosophers.
Ancient Philosophy
Medieval Philosophy
Modern Philosophy
Contemporary Philosophy
Chronological list of important philoshophers