[vl]

>how could we begin to perceive, imagine, or study existence without existence?

Through the deduction and reasoning. We don't exactly perceive sub-atomic and many galactic processes, but just infer their existence from some or other observation. Presumably "existence" is not entirely detached from "outside".

[Locke1689]

Yes but if the universe is everything that we perceive as physical, that is the material world, how could we possibly seek to examine the nonphysical world through the physical?

Consider, even reason itself is based on the rules and inherent properties of our physical universe. How can you examine a system which may not even have "reason"? Hell, our "physically" limited brains may not fundamentally be capable of comprehending such a system.

In essence, you may as well try to describe the nature of the supersensible using the sensible. It just doesn't make sense.

[vl]

Our brains comprehended a lot of stuff which appeared to be incomprehensible. Relativity theory, quantum mechanics, most branches of mathematics are so abstract that have no direct connection to the real-world "physical" experiences of our brains and bodies. And yet some members of humanity (although quite limited subset) develop these areas and reason within them. If "reason" in the widest sense is such a fundamental law that it extends beyond the whatever singularity took place, then it might still be possible to deduct something. (Simplistic example) Imagine creatures living in 2D world that itself is in 3D world and sometimes interacts with 3D object. Should these creatures be able to deduct some information about "outer" 3D world after prolonged study of projections that sometimes suddenly appear in their 2D world?