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Thales was the founder of the philosophy that all of Nature had developed from one source. According to Heraclitus Homericus (540–480 BCE), Thales drew this conclusion from the observation that most things turn into air, slime, and earth. Thales thus proposed that things change from one form to another.
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Anaximander postulated eternal motion, along with the apeiron, as the originating cause of the world. This (probably rotary) motion caused opposites, such as hot and cold, to be separated from one another as the world came into being.
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Anaximenes is best known for his doctrine that air is the source of all things. In this way, he differed with his predecessors like Thales, who held that water is the source of all things, and Anaximander, who thought that all things came from an unspecified boundless stuff.
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Pythagoras of Samos was an ancient Ionian Greek philosopher and the eponymous founder of Pythagoreanism. The Pythagorean moral philosophy is related to Orphic mysticism. It is difficult to distinguish which elements can be ascribed to the Orphics and which to the Pythagoreans. Central to this ethical philosophy was the concept of metempsychosis; the transmigration of the soul.
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In metaphysics Plato envisioned a systematic, rational treatment of the forms and their interrelations, starting with the most fundamental among them (the Good, or the One); in ethics and moral psychology he developed the view that the good life requires not just a certain kind of knowledge (as Socrates had suggested)
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Aristotle was one of the greatest philosophers who ever lived and the first genuine scientist in history. He made pioneering contributions to all fields of philosophy and science, he invented the field of formal logic, and he identified the various scientific disciplines and explored their relationships to each other.
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For Epicurus, the purpose of philosophy was to help people attain a happy (eudaimonic), tranquil life characterized by ataraxia (peace and freedom from fear) and aponia (the absence of pain). He advocated that people were best able to pursue philosophy by living a self-sufficient life surrounded by friends
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Zeno of Citium was the founder of the Stoic School of philosophy in Athens which taught that the Logos (Universal Reason) was the greatest good in life and living in accordance with reason was the purpose of human life.
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Augustine believes reason to be a uniquely human cognitive capacity that comprehends deductive truths and logical necessity. Additionally, Augustine adopts a subjective view of time and says that time is nothing in reality but exists only in the human mind's apprehension of reality.
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Aquinas believes that we should always follow our conscience, even when it is wrong or causes great harm. Since we have no way of knowing whether our consciences are wrong, they are the best guide we have as to what is the moral thing to do.
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His moral philosophy is a philosophy of freedom. Without human freedom, thought Kant, moral appraisal and moral responsibility would be impossible. Kant believes that if a person could not act otherwise, then his or her act can have no moral worth.
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Rawls's theory of "justice as fairness" recommends equal basic liberties, equality of opportunity, and facilitating the maximum benefit to the least advantaged members of society in any case where inequalities may occur.
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Jürgen Habermas seeks to defend the Enlightenment and with it an “emphatical”, “uncurtailed” conception of reason against the post-modern critique of reason on the one hand, and against so-called scientism (which would include critical rationalism and the greater part of analytical philosophy) on the other.