• Aristotle on happiness
• Epicurus on the fear of death
• David Hume on beauty
• John Stuart Mill on freedom of speech
• Friedrich Nietzsche on the death of God
• Sigmund Freud on unconscious desires
• Michel Foucault on power
• plus many more...

Featured quotes:

The unexamined life is not worth living. - Socrates, as quoted in Plato, Apology

Man is born free; and everywhere he is in chains. One thinks himself the master of others, and still remains a greater slave than they. - Jean-Jacques Rousseau, The Social Contract

Those only are happy ... who have their minds fixed on some object other than their own happiness; on the happiness of others, on the improvement of mankind, even on some art or pursuit ... Ask yourself whether you are happy, and you cease to be so. - John Stuart Mill, Autobiography

The philosophers have only interpreted the world, in various ways; the point is to change it. - Karl Marx, Theses on Feuerbach

Who can decide offhand which is absolutely better to live or to understand life? We must do both alternately, and a man can no more limit himself to either than a pair of scissors can cut with single one of its blades. - William James, Some Problems of Philosophy