For two or three years...I was a Hegelian. I remember the exact moment during my fourth year when I became one. I had gone out to buy a tin of tobacco, and was going back with it along Trinity Lane, when I suddenly threw it up in the air and exclaimed: "Great God in Boots! -- the ontological argument is sound!"
Ontological arguments proceed from a priori premises (by reason alone) to a conclusion. The first version of the ontological argument for the existence of God was made by St. Anselm in the 11th century. He claimed that it is self-contradictory to state that God does not exist:
1. God is, by definition, a being greater than which nothing greater can be imagined.
2. Existence in reality is greater than existence only in the mind.
3. God must exist in reality; otherwise, God would not be that which nothing greater can be imagined.
(Thanks to wikipedia for this simplified version of the argument.)
Compelling though this argument has been, in one form or another, to many over the years, even Russell changed his mind before too long and of course became a famous atheist. (Or agnostic, if you prefer. Russell dithered a bit about how to best categorize his position. For other philosophers, Russell felt that agnostic was the more precise term: absence of belief in god rather than presence of belief in no god. However, he agreed that for the person in the street, atheist was the more meaningful label).
Many rebuttals to the ontological argument object to the notion of existence as a property. Attackers (including Kant) argue that considering existence as a property confuses the distinction between a idea of a thing and the thing itself. Other objections criticise the "truth by definition" in premise 1.
However, my point here is not to refute the argument. I just wanted to share with you this lovely image of the young crane (Crane?)-like Russell, strolling through Cambridge on an evening long ago, thinking about this argument, and then suddenly stopping and tossing his tobacco tin into the air in excitement as an idea took hold. Isn't that gorgeous? It's one of my favorite philosophical anecdotes (SO much more pleasant than Wittgenstein allegedly lunging at Popper with that bloody poker).
P.S.: For quite a nice write-up about the ontological argument and various objections to it, see this entry from the Stanford Encyclopedia of Philosophy: