The Metaphysical Certainty of the First Way's First Premise

Parmenides argues as follows:

1. Something exists. (Premise)

2. If there were more than one thing, then they would either differ by being or by non-being. (Premise)

3. They cannot differ by being, since being is what makes them identical. (Premise)

4. They cannot differ by non-being, since to differ by non-being is to differ by nothing, and to differ by nothing is to not differ at all. (Premise)

5. Therefore, only one thing exists. (From 1 - 4)

The argument is logically valid, but it's premise (2) that is demonstrably false.  Although there have been many different attempts to circumvent this premise, I'm persuaded that Aristotle provides the best alternative.  While things are identical in their being/actuality (that-ness), they differ by their various essences (what-ness) and their varying degrees of potentialities.  Only Aristotle's Unmoved Mover is Pure Actuality, whereas other things (while participating in the actuality of the Unmoved Mover) are distinct from the Unmoved Mover due to their potentialities.  For example, an acorn is merely a type of seed in actuality, but it is an oak tree in potentiality.

I'm not going to defend the First Way in this post, with the exception of premise (1): Evident to the senses is change.  Alternatively, (1) can be stated as: Things change.

Now, why is this premise metaphysically certain?  Aristotle provides the following criticism of Parmenides's argument.  Coming to the realization that all change and all distinction is illusory itself constitutes a change, making the argument of Parmenides literally self-defeating.  We can know with certainty that things change, or at the very least, that things have the potentiality to change.

Plantinga's Modal Argument for the Immateriality of the Mind

This argument is consistent with substance dualism, hylemorphism, and a host of other philosophical views that hold that the mind is an immaterial substance.

1. There is a possible world at which my mind exists apart from my body. (Premise)

2. Whatever is possible is necessarily possible, e.g. possible in all possible worlds. (Premise, S5)

3. Hence, my mind possibly exists apart from my body in the actual world. (From 1 and 2)

4. Therefore, my mind is distinct from my body. (Implied by 3)

A skeptic's best bet to avoid the conclusion that the mind is immaterial is to reject premise (1).  However, one would be hard-pressed to find any inconsistency in the conception of the mind existing apart from the body.  Premise (2) is based on the relatively uncontroversial S5 axiom of modal logic, and (3) simply follows from (1) and (2).  (4) is implied by (3) because unless two things - A and B - are distinct, then A cannot exist apart from B.  Therefore, unless the mind is actually distinct from the body, then it would not be possible for the mind to exist apart from the body, which contradicts (1).

The Existentialist Argument

The following argument is logically valid:

1. If God does not exist, then there is no cosmic purpose. (Premise)

2. If there is no cosmic purpose, then human purpose is illusory. (Premise)

3. Human purpose is not illusory. (Premise)

4. Therefore, God exists. (From 1 - 3)

Arguments such as these will appeal to those who believe that human purpose is not something conventional or relative.  Someone like Sartre would maintain that we create our own purpose, but ultimately such purpose is illusory.  This argument is called "existentialist" because it turns Sartre's view on its head and embraces a position more akin to Kierkegaard's.  I don't maintain this is a proof or demonstration of God's existence, but I do think it constitutes one of many rationally acceptable reasons to believe in cosmic purpose and, ultimately, God, the giver of purpose to the cosmos.