I have flirted with the idea of formulating a Thomistic argument that is purely a priori. More than one comes to mind, but I am interested in exploring a reductio ad absurdum argument that establishes the existence of a necessary being.
1. If nothing exists, then possibility does not exist. (Premise)
2. If possibility does not exist, then it is not possible for nothing to exist. (Premise)
3. Hence, something necessarily exists. (From 1 and 2)
4. Possibly, nothing contingent exists. (Premise)
5. Therefore, something necessary exists. (From 3 and 4)
6. Whatever is necessary has an essence identical to its existence, e.g. it is pure actuality. (Premise)
7. Therefore, pure actuality exists. (From 5 and 6)
The argument is clearly valid, but is it sound? If it is, then we have a sound a priori argument for the existence of God as being itself subsisting.