One prevalent arguments, though one that it seems does not convert many people, is the ontological argument. The ontological argument essentially says that god exists because it is his nature to exist. More formally the argument can be summarized as:
-God is defined as the being that is perfect in every way
-not existing is a form of imperfection
-therefore god cannot not exist
-therefore god must exist
Essentially what is being done in this argument is that god is defined to exist and then from this definition concludes that god must exist. This at first appears to be a simple circular argument, assuming god exists and then using that to prove that he exists, and in a way it is, but this brings up the question “why can’t we define a being however we like to?”
There are two things to be said here, one is that the argument can be made that whether or not a being exists is not a property of that object but rather a property of reality, thus a being’s definition should not include whether or not it exists.
The second point that can be made is that there is a difference between defining a beings and trying to define a being. A group of words in the English language, or any other language, can either define a being or not. Of the groupings that don’t define a being this can be either obvious or non obvious, for example it is obvious that ‘the is what am or but’ does not define a being. Consider the phrase “a square circle” does it define an object? It seems like it should upon first glance, when reading it many of you probably thought of something that looks like a square overlapping a circle, but that’s not a square circle, an object is square iff it has 4 equal straight sides and 4 right angles, an object is a circle iff it is the locus in a plane of all points equidistant from one point, the set of objects that are circles and the set of objects that are square don’t intersect (unless you count the trivial case of a point but that can be easily dealt with by changing the phrasing to “a nontrivial square circle”). that means that there is no object that is a square circle so ‘a square circle’ doesn’t define an object it only appears to. Now suppose that god does not exist, then the set of beings that have the other qualities of god and the set of beings that exist are non intersecting and thus the definition of god given at the beginning of the ontological argument doesn’t actually describe a being. The argument relies on the implicit assumption that a being has been defined and is thus rendered invalid by this.
Michaeldruggan's Blog 