This is an overview/refresher of major philosophers of the Enlightenment.
Rene Descartes (1596-1650)
Baruch Spinoza (1632-1677)
John Locke (1632-1704)
Gottfried Leibniz (1646-1716) was a German philosopher and mathematician (who invented infinitesimal calculus, concomitantly with Newton). Along with Descartes and Spinoza, Leibniz was one of the great seventeenth century rationalists.
Leibniz addressed the theodicy question (if God is all good, all knowing, and all powerful, why is there evil and suffering in the world?) by explaining that God, in creating the world, choose, from among a set of all possible worlds, the best possible world, much like a mathematician might find the curve of shortest length (geodesic) between two points. Voltaire satirized Leibniz's idea in Candide.
Leibniz is also know for his somewhat eccentric metaphysical system enumerated in the essay "Monadology". Everything in our world is made of monads. A monad is a single, atomic, indivisible thing. A monad has no parts, but it does have qualities. While all monads are quantitatively the same (same size, etc.), each individual monad is qualitatively different from all other monads—each monad is unique, unique in itself not just in its position in relation to other monads. Monads can change, but that change must come from inside the monad; no external force—short of divine intervention—can change a monad. In fact, a monad contains within itself, at all times, all the qualities it has exhibited in the past, and all properties it will exhibit in the future. (Leibniz evocatively says that, at each moment, a monad is "pregnant" with its immediate future.)
Since monads are fully self-contained and can not effect each other, and everything is made of monads, the appearance of cause and effect in the world is an illusion. Leibniz offers an explanation in terms of two clocks in the same room. Both clocks keeps perfect time, and strike the hour at the same instant. A naive observer might suppose that both clocks are synchronized by some joining mechanism, but the clocks are simply set to the same time. If you moved the clocks for different rooms they would continue to strike at the same instant. If you stopped one clock, the other would continue to run. The clocks are not causally linked, and any appearance to the contrary is false.
That things in the world appear to interact is due to the fact that God created them with pre-established harmony. Each monad is programmed with its relation to every other monad. Point 56 in "Monadology" has always struck my as conceptually beautiful:
The interconnection of each thing to everything else happens because each monad has qualities that express its relation to all the others, so each monad is a perpetual living mirror of the entire universe.
Have you seen those Buckyballs desk toys? They're rare earth magnets shaped as spheres about the size of BB gun pellets. They come packaged in a cube arrangement. I imagine Leibniz's universe like those—each ball as a monad, a perfectly polished optical surface reflecting every other ball.
So how does Leibniz's philosophical system benefit from this unusual monad theory?
George Berkeley (1685-1753) was an Anglo-Irish philosopher who created a philosophy of perception which came to be known as subjective idealism—that one's knowledge of the world is limited to sensations, which are themselves ideas in one's mind. Therefore, the objective, material world of non-mental matter (if such a world exists at all) is unknowable and empirically unprovable. Berkeley's subjective idealism is a response to Locke's materialism.
Berkeley accepted a consensus reality, explaining that everything (all minds and ideas) existed within the mind of God. God is also how Berkeley explains object permanence—a chair continues to exits when you're not looking at it, because God still sees the chair, as illustrated in this well-known pair of limericks:
There was a young man who said "God
Must find it exceedingly odd
To think that the tree
Should continue to be
When there's no one about in the quad."
"Dear Sir: Your astonishment's odd;
I am always about in the quad.
And that's why the tree
Will continue to be
Since observed by, Yours faithfully, God."
(Incidentally, the first limerick was written by Robert Knox, perpetrator of the Broadcasting form the Barricades hoax. The author of the second limerick remains anonymous.)
Berkeley summarized his philosophy as "esse est percipi"
(to be is to be perceived).
Jorge Luis Borges' short story "Tlön, Uqbar, Orbis Tertius" plays with Berkeleian idealism.
Hume (1711-1776) was a Scottish philosopher and historian. Along with Locke and Berkeley, Hume was considered one of the major British Empiricists.
Hume explored the application of contemporary scientific methodology (i.e.—Newtonian physics) to philosophy, and whether human knowledge (of what is true or false) is limited to what one can experience. Hume's most important contribution to epistemology (and perhaps the basis of his entire doctrine) is the Problem of Induction, which questions whether inductive reasoning can lead to empirical knowledge. Hume observes that inductive reasoning is based on two premises of questionable justification:
Hume concludes that there is no rational justification for inductive knowledge, but that Nature imbues man with an instinctual custom of belief which prevents him from falling victim to radical skepticism. (A radical skeptic might starve to death because they could not infer the benefits of food based on previous observations of nutrition, whereas a practical skeptic would eat because of a natural instinct to accept the inevitability of induction.)
This anti-rationalism leads Hume to conclude that:
Humes major works include A Treatise of Human Nature, An Enquiry Concerning Human Understanding, and An Enquiry Concerning the Principles of Morals.
Jean-Jeacques Rousseau (1712-1778)
Immanuuel Kant (1724-1804)