RomanJoe wrote:

My first concern is that one could object that essences, distinct forms, substances, don't exist.

If essences don't exist then my question would become how do you and I understand the quiddity of a thing?

RomanJoe wrote:

All there is is the entirety of the universe's particles.

If all there are just particles then how can life come out of non-life (not a scientific question)? 

RomanJoe wrote:

Everything is matter in motion.

Then we need an unmoved mover.

RomanJoe wrote:

Nothing is truly distinct or separate from each other in this monist understanding of the world.

If nothing is truly distinct and separate from each other then you should probably be able to understand me without me talking to you and making an effort to know you as a person (I hope I am not coming off as polemical).

RomanJoe wrote:

My second concern is that the PSR or the PC may not be absolute and that perhaps what grounds the universe is just an unexplained sea of fluctuating energy or some type of basic prime matter. Perhaps the prime mover has unexplained potencies, perhaps the prime mover is material.

Even if it is a sea of fluctuating energy and some basic prime matter then we still need a mover outside of it that needs to actualize the energy and the prime matter so we are back to a first mover.

Give that there is only one first cause, then If the first cause has unexplained potencies then those potencies need to be actualized by the first cause itself which means that the first cause would need to have it as actual to be able to do so and that leads to absurdity.
 

RomanJoe wrote:

But if the PSR and PC aren't absolute, then all of this would be undermined. The bedrock of existence could be inexplicable. Also, wouldn't an atomist perception of the world, where everything is reduced to particles, completely undermine essences? 

I think an atomist theory would undermine essences. The question is whether the atomist theory is true. Quite unlikely in my view.

RomanJoe wrote:

But if the PSR and PC aren't absolute, then all of this would be undermined. The bedrock of existence could be inexplicable. Also, wouldn't an atomist perception of the world, where everything is reduced to particles, completely undermine essences? 

Sorry I missed your comment there. Then the burden of proof is on the atomist to give us an explanation of why there is life out of non life, intellect out of just non intellect. This then needs to be a metaphysical question rather than a scientific one and the metaphysics of atomist needs to be spelt out in order for us to get any where.