1. “The unexamined life is not worth living” – Socrates (470-399 BCE)
Socrates’ [wiki] belief that we must reflect upon the life we live was partly inspired by the famous phrase inscribed at the shrine of the oracle at Delphi, “Know thyself.”
Through his relentless questioning, he forced people to examine their own beliefs. He saw the citizens of his beloved Athens sleepwalking through life, living only for money, power, and fame, so he became famous trying to help them.
2. “Entities should not be multiplied unnecessarily” – William of Ockham (1285 - 1349?)
Commonly known as Ockham’s razor, the idea is that judging among competing philosophical or scientific theories, all other things being equal, we should prefer the simplest theory.
3. “The life of man [is] solitary, poor, nasty, brutish, and short.” – Thomas Hobbes (1588 – 1679)
Referring to the original state of nature, a hypothetical past before civilization, Hobbes [wiki] saw no reason to be nostalgic.
Whereas Rousseau said, “Man is born free, and he is everywhere in chains,” Hobbes believed we find ourselves living a savage, impossible life without education.
4. “I think therefore I am” – René Descartes (1596 – 1650)
Descartes [wiki] began his philosophy by doubting everything in order to figure out what he could know with absolute certainty. Although he could be wrong about what he was thinking, that he was thinking was undeniable. The inference is that those who do not think do not exist.
5. “To be is to be perceived (Esse est percipi).” Or, “If a tree falls in the forest and no one is there to hear it, does it make a sound?” – Bishop George Berkeley (1685 – 1753)
As an idealist (the ultimate subjectivist), Berkeley [wiki] believed that nothing is real but minds and their ideas. Ideas do not exist independently of minds.
Though he never put the question in the exact words of the famous quotation, Berkeley would say that if a tree fell in the forest and there was no one (not even a squirrel) there to hear it, not only would it not make a sound, but there would be no tree.
6. “We live in the best of all possible worlds.” – Gottfried Wilhelm Leibniz (1646 – 1716)
Voltaire’s famous novel Candide satirizes this optimistic view. And looking around you right now you may wonder how anyone could actually believe it.
The principle of sufficient reason holds that for everything, there must be sufficient reason why it exists. And according to Leibniz the only sufficient reason for the world we live in is that God created it as the best possible universe. The unstated assumption is that God is good.
7. “The owl of Minerva spreads its wings only with the falling of the dusk.” G.W.F. Hegel (1770 – 1831)
Similar to “vision is 20/20 in hindsight,” Hegel’s [wiki] poetic insight says that philosophers are impotent. Only after the end of an age can philosophers realize what it was about. And by then it’s too late to change things. It wasn’t until the time of Immanuel Kant (1724 - 1804) that the true nature of the Enlightenment was understood, and Kant did nothing to change the Enlightenment; he just consciously perpetuated it.
Marx (1818 – 1883) found Hegel’s apt description to be indicative of the problem with philosophy and responded, “the philosophers have only interpreted the world differently, what matters is to change it.”
8. “Who is also aware of the tremendous risk involved in faith – when he nevertheless makes the leap of faith – this [is] subjectivity … at its height.” – Søren Kierkegaard (1813 – 1855)
In Kierkegaard’s [wiki] theory of stages of life, the final stage, the religious stage, requires passionate, subjective belief rather than objective proof, in the paradoxical and the absurd.
Faith has its rewards, but it isn’t rational. It’s beyond reason. As Blaise Pascal said, “The heart has its reason which reason does not know.”
9. ““It is the obligation of the intelligent to oppress the stupid, otherwise they will take over the world.” – Albert Camus (1913 – 1960)
Camus’ [wiki] solution to the philosophical problem was to recognize and embrace life’s absurdity.
For Camus, the absurd hero is Sisyphus, a man from Greek mythology who is condemned by the gods for eternity to roll up a stone up a hill only to have it fall back again as it reaches the top.
10. “One cannot step twice in the same river.” – Heraclitus (ca. 540 – ca. 480 BCE)
Heraclitus definitely isn’t alone here. His message was that reality is constantly changing it’s an ongoing process rather than a fixed and stable product. Buddhism shares a similar metaphysical view with the idea of annica, the claim that all reality is fleeting and impermanent.
11. "A witty saying proves nothing."
Voltaire (wiki) was known for his wit and in this statement denies his followers' the right to applaud when a truth is delivered with acerbity.
http://www.neatorama.com/2007/02/06/11-most-important-philosophical-quotations/