Today is the feast day of St. Thomas Aquinas. Here is a bit of his writings:
Demonstration of God's Existence
I answer that, The existence of God can be proved in five ways.
The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.
The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence--which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.
The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like. But "more" and "less" are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaph. ii. Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God.
The fifth way is taken from the governance of the world. We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.
(Summa Theologica, I, q. 2, art. 3)
Here is a spoof, of sorts:
THE FIVE WAYS OF PROVING SANTA CLAUS*
(from Joseph Magee's Thomistic Humor Page)
Whether Santa Clause Exists
We proceed thus to the third article:
It seems that Santa Claus does not exist; because Christmas gifts are able to be given by good elves. Therefore, Santa Claus does not exist.
Further, if Santa Claus did exist, there would be no narrow chimneys. But there are narrow chimneys, and sometimes there are no chimneys at all. Therefore, Santa Claus does not exist.
On the contrary, Kay Starr says: "I saw Mommy kissing Santa Claus underneath the mistletoe last night."
I answer that, The existence of Santa Claus Can be proved in five ways.
The first and most manifest way is that taken from Christmas trees. It is certain and evident to our senses, that in the world some things are Christmas trees. Now no pine tree becomes a Christmas tree unless it is trimmed. Now to be trimmed means to receive ornaments from another. But this cannot go on to infinity in the trimming of Christmas trees. One must come to some first untrimmed trimmer; and this everyone understands to be Santa Claus.
The second way is from the nature of Christmas gifts. We see that in the world that Christmas gifts are given and received. Whoever, then, gives Christmas gifts either receives them from another or makes them in his workshop. If, however, no one makes Christmas gifts in his workshop, they are not given nor received. Therefore it is necessary to posit some first giver of Christmas gifts, who everyone calls Santa Claus.
The third way is taken from plastic images resembling Santa Claus. At all stores we see things of plastic that represent Santa Claus. These things are of such a quality that they are representations according to Santa himself or according to other images of him. But, it is not possible to proceed to infinity in images. Therefore, it is necessary to posit something which is resembling Santa Claus and hence Santa Claus exists.
The fourth way is taken from the grades which are found in Christmas spirit. Indeed, in this world, among men there are some of more and some of less Christmas spirit. But "more" and "less" is said of diverse things according as they resemble in their diverse ways something which is the "maximum." Therefore there must be something which has the most Christmas spirit, and this we call Santa Claus.
The fifth way is taken from the behavior of children. When Christmas day approaches, we see from their being good always or frequently that children, who lack understanding, are moved because of an end. But children would not be good because of the Nativity of Christ unless there were someone who strengthened them so that they were good. And this someone is known by all to be Santa Claus.
Replies to Objections:
Good elves, since they receive Christmas gifts from another, should be named the highest helpers of Santa.
It is not impossible that Santa Claus should use the door like everyone else.
* Also known in various places under the name of Saint Nicholas, Father Christmas, etc.