Why did Plotinus associate matter with evil?

For Plotinus, the universe is characterized by a process of expansion from the One, or the Good, through diminishing degrees of reality. This expansion eventually reaches an absolute limit where the presence of being ceases entirely.

Plotinus identified this final point of total absence as matter. Because matter represents the ultimate negation of the Good—the source of all reality and excellence—he defined it as the principle of evil. While his language regarding matter often carries a dualistic tone, he did not view it as an independent power opposing the Good; rather, he understood it as the terminal point of the creative process where reality fades into darkness.1

It is important to note that Plotinus did not hold a dualistic view of the material universe as a whole. He maintained that the physical world is a beautiful, organic reflection of the intelligible realm. He regarded individual earthly bodies negatively only insofar as they act as a hindrance to the soul's intellectual and spiritual ascent toward the One.1