Before we get started, there are two important words that must be known.
Metaphysics; meaning trying to figure out what reality really is.
Epistemology; meaning the theory of knowledge and how we know what we know.
Now on to Descartes; people look at Descartes as the beginning of modern philosophy. He follows from the scholastic/medieval period, where learning was simply about Aristotle and the Bible.
"I think, therefore, I am" is one of the most famous lines in philosophy, formed by Descartes. It forms the cogito, pronounced co-ji-toe.
Towards the end of 17th century and the beginning of 18th century a debate between was sparked between empiricists and the idealists i.e. British empiricism and continental rationalism.
Empiricists' idea of knowledge was that 'we know what we know because we can see it'.
Locke, a famous empiricist philosophy, has no innate ideas, all his knowledge was gained through experience. He believed we only get knowledge through experience and it all comes from senses; sensory data. His philosophy insisted there are no innate ideas; when you are born, it is a blank slate, there is nothing in your mind and is absent of any ideas. He completely disagreed with Plato's metaphysics.
Rationalists'/Idealists' idea of knowledge was that 'you can learn about the world simply by thinking about it'. Sensory data is not needed for this. Logical answers can be formed in the mind; the pre-eminence of the mind. Descartes, Leibniz and Spinoza are all examples of idealists.
Materialists' idea of knowledge is that only material exists. It is the idea that everything is either made of matter or is dependant on matter for existence. Hobbes is a well-known materialist philosopher.
What is going on in the world outside my mind?
The issues he raised were right at the start of philosophy, at the same time as Locke, Shakespeare, Galileo. Despite being a man of God, he was very much in favour of science and was critical of the education he recieved at university such as Aristotleanism and traditional education. Artistotlean logic was circular as everything was based around two books and Descartes said: "I gained nothing but an increasing recognition of my ignorance". He felt all he knew and all his education was useless and a waste of time.
After education, he took it back to basics and assumed everything he knew was wrong and decided to start all over again. This project was the method of doubt, known as 'Cartesian doubt'. A famous example of doubt is "If you have a barrel of apples and fear one of them is rotten, empty them all out and check if everyone one of them is rotten or not". This is what Descartes did with knowledge. He wanted to create a theory of knowledge that would stand the test of time.
You have to accept your almost certainities may be uncertain. It is at least possible your name may not be your name, your parents may not be your parents, your age may not be your age; after all, you could be hypnotised. It's very unlikely but it is possible. Senses have before deceived you, for instance, when drunk, when tired and when lost. If senses have even let you down once, you have to dismiss them, you can not trust them.
Descartes then comes to a moumental moment of accepting all may be untrue. However, he says 'I am thinking, this means that I exist'. It is from this thought the phrase cogito ergo sum was formed; I think, therefore, I am. Even if my thoughts are wrong, I am still thinking, therefore I still exist.
He identifies his thinking with his mind; the mind and the body are entirely seperate. This is Cartesian dualism. If someone was to cut off my hand, I'd still exist, therefore my existance depends on my mind, not my body, therefore, they are clearly two seperate things. He can still doubt everything except his mind.
Berkeley has a similar view to Descartes, that there was only one substance and that is mentality. Berkeley said "all we can be sure of is that my mind is telling me these things and it is real to me; it is impossible to prove anything out of this"
Decartes' epistemology sets off a tendency in European philosophy called idealism and included philosophers such as Kant and Hegel.
After getting to this point, he knows he needs to escape the Cogito. He needs to avoid collapsing into solipsism; only I exist for sure. His answer was God. He needs God to help him climb out and escape into the Cogito. In order to do this, he has to prove God exists, for which he came up with three arguments.
Descartes has the idea of God; so where did it come from? It can't have come from no where; God must have given it to him. God is a benevolent being and therefore wouldn't deceive us.
A priori argument for God's existence. God is a perfect being, so he must exist, if he didn't exist, he wouldn't be perfect.
Everything has a cause, at some stage something must have triggered the first cause; the uncaused cause; this being God.
He argues that if the world is seperate from God then he has boundaries, so for God to be infinite he has to be part of the world. Therefore there was only one substance, not two like Descartes says, this was God. This was monism or pantheism; there is one thing and that thing is God - as opposed to dualism. All our thinking is embodied and has a physical manifestation.
Leibniz thought the world was made out of one type of thing called a Monad; a simple, non divisible, soul like entity that is everything" His philosophy cuts everything down to the minimal. As long as something still has an extension, it can still be divided. The smallest thing something can be before physical being is a Monad; it is the building block of everything. It is similar to Plato's forms.
Huxley was a believer in epiphenomenalism - the idea that mental events are caused by physical events in the brain, but have no effects upon any physical events; physical beings, closed causality, physical brains. He believed our inner lives were just the hum of the machine but doesn't move the machine; it's just a by product. He compared mental events to a steam whistle that contributes nothing to the work of a locomotive.