Plotinus and other Platonists identified matter with evil. Plotinus scholar, Lloyd P Gerson, explains their dualism in his pivotal work Plotinus:
We have already encountered briefly Plotinus’ identification of matter with evil. Such an identification, along with the identification of goodness with the first principle of all, gives coordinates as it were by which to judge moral progress and decline. These can be described alternatively as approaching one terminus and distancing the other. Just as approaching the Good means incremental identification with noetic activity, so declining towards evil means incremental identification with things contaminated with evil.