The text below was originally written for Quora.com but in writing it I decided it has a place here as well.
Descartes is often described as a rationalist philosopher. This is because he explains that empirical data, data of the senses, is inherently doubtful. What you take to be a sensation of the external world could be generated inside your head and there is no way to tell the difference.
Descartes reasoned that you cannot doubt your own existence: you cannot doubt that you are doubting. However this is not enough to vouch for a world outside yourself. This is why he needed an external guarantor: God.
He then needed a non-empirical proof that there is such an external guarantor. Personally, I find it hard to accept that he was wholly serious about the ontological argument as it uses the kind of medieval philosophy that Descartes brilliantly debunks and thereby reconfigures Philosophy. Descartes argues that the idea of God necessarily entails existence, for without existence the concept would be something less than God. The medievalists had said that lack of existence would be an imperfection and therefore could not be an attribute of God. Sometimes I wonder if Descartes included this argument to keep the church happy. He had seen the fate of Galileo and others who had questioned official teaching and had moved to the Netherlands to escape the church’s reach. However there is another reason that I explain later.
The interesting ‘proof’ is his rationalist version of the first cause argument. Normally this is an empirical argument: observed events have causes, which are themselves events that also have causes; God is the originator of the causes (how God is causeless is an obvious inconsistency). Descartes argues that ideas must have causes and the chain of causes of ideas must start somewhere.
This is novel, but apart from being open to the objections of the empirical first cause argument, it leaves the first cause as a conceptual and immaterial entity. This is why he needs an argument for existence. For Descartes existence is both material and of ideas, he is a dualist, so whether his conception of God’s existence is in some way material does not matter so much.
Viewed from four centuries later, Descartes’ guarantor of an external world looks more like a provisional hypothesis than a point of certainty, however this is less important than it might seem. What matters is that a guarantor of external reality enables Descartes to construct foundations for knowledge and a new approach to Science and understanding.
Read Descartes’ Méditations, it is a short work, but one of the most remarkable and revolutionary texts ever written. It can be read as Philosophy and as literature: an extraordinary modern novella. Although originally in Latin, Descartes oversaw the French translation; if you can read it in French, do.