Hello, Jim,
These are fascinating philosophical reflections to contemplate on! I see remarkable potential in this minimalistic meditation on existence. The core metaphysical questions you're exploring - about thought, space, and being itself - are compelling. To further develop these themes while maintaining your spare style, I'd suggest a revision that deepens both the philosophical resonance and rhythmic flow - feel free to take or leave as you see fit!
Existence
A thought slips past
through boundless space
seeks form when cast
but finds no place
there where to be
while not to be
This revision preserves your poem's essential exploration of existence while adding layers that engage with classical questions about the nature of thought and being. The movement through boundless space suggests infinite possibility, while the seeking of form echoes philosophical inquiries about how thoughts manifest in reality. The final couplet creates an intriguing temporal and spatial paradox while maintaining your thoughtful nod to Shakespeare.
Good luck with this fascinating exploration, Jim!
Cheers,
...Alex