n. Any system of thought which seeks to deduce all the varied phenomena of both the physical and spiritual worlds from a single principle; specifically, the metaphysical doctrine that there is but one substance, either mind (idealism) or matter (materialism), or a substance that is neither mind nor matter, but is the substantial ground of both: opposed to dualism.n. Any theory or system which attempts to explain many heterogeneous phenomena by a single principle.n. In biology, same as monogenesis .