MN 1: Mūlapariyāya Sutta (The Root of All Things)
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A challenging discourse (even for those who first heard it!), this first sutta of the Majjhima Nikaya is a forceful rejection of all forms of monism, and the Samkhya philosophy in particular.
For a translation of this sutta’s semicanonical commentaries, see Bhikkhu Bodhi’s The Root of Existence.