It is reasonable to believe that Santa Claus exists.

== The Argument == (1) Possibly, Santa Claus exists. (premise) (2) If modal realism is true, necessarily, all possible truths obtain in one or more possible worlds. (definition) (3) Modal realism is true. (premise) (4) Possibly, necessarily Santa Claus exists in one or more possible worlds. (from 1,2,3) (5) Necessarily, Santa Claus exists in one or more possible worlds. (from 4, S5 axiom) == Brief Defense of the Premises == P1: Possibly, Santa Claus Exists. The proposition that "X is not logically possible" is an alethic modal claim, ie., that there is no possible world wherein X obtains.[1] Square circles or married bachelors are examples of things that logically cannot exist in any possible world. Stated simply: (1) Any proposition 'p' is possibly true just in case 'p' can be asserted without implying a logical contradiction. (2) Santa Claus's existence can be asserted without implying a logical contradiction. (3) Therefore, it is possibly true that Santa Claus exists P2: If modal realism is true, necessarily, all possible truths obtain in one or more possible worlds. This follows from the definition of modal realism, which is the thesis that our world is just one among many like it. All possible worlds are as real as the actual world. The term "actual" world is indexical, that is, it merely designates that a particular world is *our* world. A proposition, 'p' is possibly true just in case 'p' is true in one or more of these worlds.[2] P3: Modal realism is true. This is admittedly the most controversial of my starting premises. I'll support it using three separate lines of reasoning. First, it is a well known fact that the parameters of the physical constants of the universe must fall within extremely precise ranges in order to permit life. According to Stephen Hawking, if the relationship between the competing effects of explosive expansion and gravitational contraction during Plank time deviated in their ratio from unity by just one part in 10e60, the universe would have either re-collapsed in on itself or expanded too rapidly for stars to form.[3] One potential explanation for how the conditions of this incredibly slim possibility were met is the existence of other possible worlds. Although there is no reason to prefer this theory over others, it is at least plausible and can't be readily disproved. Second, modal realism is parsimonious. An extremely large, but finite set of alternate worlds could explain the existence of sentient life, however, as per the Principle of Sufficient Reason, the fact that some worlds exist but others don't would require an explanation. The hypothesis that all possible worlds are actual worlds is simpler. Finally, it is reasonable to believe that modal realism is true irrespective of all other evidence. It is common in mathematics to postulate that abstract entities, such as sets, exist as real objects simply because they are useful constructs. Since there is no reason to doubt the existence of mathematical sets, they can be accepted uncritically for pragmatic reasons.[4] The same is true of modal realism, which is useful for understanding possibility, probability, contingency, necessity, counterfactuals and other important concepts in epistemology, metaphysics, and philosophy of mind. Remember, don't major in philosophy, kids, or you'll wind up like this! David Lewis is either laughing his arse off right now or turning in his grave. Hell, maybe he's laughing in one possible world and turning in his grave in another! Merry Christmas, folks. The resolution is affirmed. References: 1. http://en.wikipedia.org... 2. http://plato.stanford.edu... 3. Neil A. Manson, God and design: the teleological argument and modern science (Psychology Press, 2003) p. 6. 4. http://plato.stanford.edu...