(Originally) supposed particles of mental substance in combinations which are perceived as matter; (in later use also) any rudimentary abstract substance from which ideas, images, etc., can be formed.
Origin
Late 19th century.
Definition of mind-stuff in US English:
mind-stuff
noun
(Originally) supposed particles of mental substance in combinations which are perceived as matter; (in later use also) any rudimentary abstract substance from which ideas, images, etc., can be formed.