monopsychism
/ˌmɒnə(ʊ)ˈsʌɪkɪz(ə)m/Metaphysics
noun
The theory, found in Neoplatonic thought and in the Averroistic school of medieval Aristotelianism, that there is but one immortal soul or intellect in which all humans somehow share.
Origin
Mid 19th century. From mono- + psychism. Compare earlier polypsychism.