"Letzen Endes wird also der Materiebegriff in beiden Fällen auf Mathematik zurückgefürt. Der innerste Kern alles Stofflichen ist für uns wie für Plato eine Form, nicht irgendein materieller Inhalt."
"In the final analysis, in both cases [Plato and modern physics] the notion of matter is essentially a mathematical concept. The most fundamental kernel of all that is material is for us, as well as for Plato, a [mathematical] form, and not some material content."
— W. Heisenberg, "Platons Vorstellungen von den kleinsten Bausteinen der Materie und die Elementarteilchen der modernen Physik," Im Umkreis der Kunst. Eine Festschrift für Emil Preetorius , Wiesbaden 1953, pp. 137-140, as cited by Luc Brisson and F. Walter Meyerstein in Inventing the Universe , SUNY Press, 1995.
See also remarks by Pauli in For All Hallows Day.
Update of 1 PM
Related material —
"Zweiteilung und Symmetrieverminderung, das ist des Pudels Kern. Zweiteilung ist ein sehr altes Attribut des Teufels."
—Pauli to Heisenberg
Here "the poodle's kernel" is a reference to Faust , where the devil appears as a poodle.
On Schopenhauer's later years—
"In Frankfurt he spent the remaining years of his life, living quietly in two rooms with his pipe and his flute but with no friends or companions except a small poodle, the only creature to which Schopenhauer ever seems to have felt any real attachment. He named the dog Atman, a term taken from the pessimistic religion of India, in which Schopenhauer had become more and more interested in his later years."
— Robert F. Davidson, "Pessimism: Arthur Schopenhauer" in Philosophies Men Live By , New York, The Dryden Press, 1952