Aristotle. In short: the man. The brainy man. Greek philosopher, student of Plato, teacher of Alexander the Great, and one of Western thought's most important figures. His writings were the first to create a comprehensive system of Western philosophy - morality and aesthetics, logic and science, politics and metaphysics - and yet only a third of them survived. He covered physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology. His understanding of physical sciences lasted until Isaac Newton's apple dropped, and we still talk about his philosophy today. There's an excellent Wikipedia on him here, but below are some of my favourite sayings of his.
Love is one soul in two bodies.
A tyrant must put on the appearance of uncommon devotion to religion. Subjects are less apprehensive of illegal treatment from a ruler whom they consider god-fearing and pious. On the other hand, they do less easily move against him, believing that he has the gods on his side. All human actions have one or more of these seven causes: chance, nature, compulsions, habit, reason, passion, desire.
All men by nature desire knowledge. All paid jobs absorb and degrade the mind.
All virtue is summed up in dealing justly.
Anybody can become angry - that is easy, but to be angry with the right person and to the right degree and at the right time and for the right purpose, and in the right way - that is not within everybody's power and is not easy.
Character may almost be called the most effective means of persuasion.
Courage is a mean with regard to fear and confidence.
In nine cases out of ten, a woman had better show more affection than she feels.
It is best to rise from life as from a banquet, neither thirsty nor drunken.