a mind really sincere and thoroughly purified is the necessary preliminary to the understanding of reality in its suchness… this came later to be formulated by the Mahāyānists into the doctrine of Thatness or Suchness (bhūtatathatā).
Suzuki grounds suchness in early Pali epistemology and traces its formal doctrinal elaboration into Mahāyāna as bhūtatathatā, framing it as the cognitive goal of purified, non-dual understanding.
, Essays in Zen Buddhism (First Series), 1949thesis