Plato conceived of an
ultimate reality and truth, to which he gave the name “Form.”
-Dr. David Roochnik, professor of philosophy, Boston University , Introduction to Greek Philosophy, course guidebook
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Plato’s ontological and epistemological teaching is that with the right education, one possessing the proper constitution can advance from mere opinion of sensible particulars to true knowledge of
the eternal, unchanging Forms—i.e., of ultimate reality."
- Dr. Michael Sugrue, professor of philosophy, Princeton University, Plato's Republic, course guidebook
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Hindu scriptures describe
ultimate reality as Brahman. Brahman is non-dual pure consciousness, indivisible, incorporeal, infinite, and all-pervading like the sky."
https://ramakrishna.org/theultimatereality.html
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Immanuel Kant did not doubt that the world as it appears to us
is not the ultimate reality. He distinguished between phenomenal reality, or the world as we experience it, and
noumenal reality, which is purely intelligible, or non-sensual, reality."
https://faculty.mtsac.edu/cmcgruder/partiikant.html