I think, therefore I am..

Western Philosophy in the 17th and 18th centuries was divided between British empiricism and continental rationalism. These theories are concerned with Epistemology (science of knowledge.) 

Empiricism 

The only source of knowledge is the senses. 

Locke believes that there are no innate ideas and that everyone is born as a blank slate. The knowledge we gain he says is through experience.

Bacon believes in the scientific method- avoid idols of the mind.

Plato believes that everyone is born with knowledge - perfect forms.

Rationalists 

Descartes, Leibniz and Spinoza all believe in pure reason, the mind alone or at least the pre-eminence of the mind.

Metaphysics

Only material exists (substance without mind)

Idealism

Deny existence of matter; everything is ideas

When I look at a table, I am not aware of the table, but what the table looks like to me - it is not necessarily a table, but an effect which is produced in my mind when I look at it. This is the same for touching, tasting, etc.

Descartes

Descartes was a part of the 17th century and was contemporary to Galileo. He shared common views with both Bacon and Galileo as they opposed to Aristotelianism and traditional education available in universities. He said 'I had gained nothing but an increasing recognition of my ignorance.' He went travelling around Europe fighting wars and hoped that a life of action would give him an incite. He was disappointed and made an ambitious plan to search for true knowledge. He rebuilt human knowledge using knowledge that had been tested and were certain (unquestionable.) The project was called 'the method of doubt' (Cartesian doubt.) Descartes dismisses any knowledge for which there are grounds for doubt except the existence of God, it doesn't matter if these doubts make you feel doubtful, as your senses often deceive you anyway. The mind is one thing - the body another; this is Cartesian dualism. 

Cartesian philosophy contains three realms - the mind/soul, matter and God. The mind/soul and matter are created substances and God is the uncreated substance. Mind is a thing in itself - Hagel. Descartes' epistemology set off a tendency in European philosophy called idealism. He has the idea of a perfect being and believes that God is perfect, his thought of God proves the presence of God.

The Ontological Argument

Descartes needed this argument to prevent the Cogito (Cogito Ergo Sum - I think, therefore I am) from collapsing into solipsism (idea that one's mind is sure to exist, but everything else may not). 

Kant (idealist) believe that existence is a necessary condition for thought- not a result. This idea was rejected by the materialists as they believe that matter is fundamental and thus the 'hard problem' is created. This is 'how do you account for consciousness?'

Spinoza

In a state of nature there is no right or wrong. He believes there is only one substance unlike Descartes that believe there is two. He decided that God or nature is the true substance. He disagrees with dualism as he thinks that everything is part of some substance. God did not create he is nature and people are a part/ aspect of God. This eliminates sin and evil as everything that happens is simply a manifestation of God.

Romanticism is a non- personified God, nature or spirit. His chief work was The Ethics that discusses metaphysics, psychology of the passion and will (psychology is reminiscent to Hobbes but ethic is original.) He also believes that freedom of opinion is important and believes that the Sovereign can do wrong agreeing with Hobbes that the church should be subordinate to the State.

Leibniz

He believes in the notion of 3 substances like Descartes and Spinoza - God, mind and matter. He argues that we live in the best of all possible worlds, because God is good and omnipotent, and he created/chose this world from an infinite number of possibilities. Influence on his philosophy was that of Spinoza as he had metaphysical proofs of Gods existence. He invented infinitesimal calculus in ignorance of Newtons previous but unpublished work on the same subject. However this did not explain why God created evil.