Leibniz's monadology is pluralism pressed to its logical extreme: the universe, he argued, is built from an infinity of simple, indivisible substances called monads, each with no windows onto the others and no direct causal contact between them, their apparent coordination explained instead by a pre-established harmony set at creation. This is pluralism about substance in the strongest sense, since even the appearance of one shared physical world is, on this view, a byproduct of many separate, non-interacting perspectives.
William James took a much less exotic but equally firm pluralist line two centuries later, arguing directly against the monist assumption, common to both idealists and materialists of his day, that reality must ultimately be one integrated whole. For James, the universe is genuinely unfinished and only partly unified, strung together rather than seamlessly fused, a picture he thought did better justice to novelty, chance, and real moral effort than any monism could. Aristotle is sometimes claimed as an ancestor of the position too, for treating substances, individual things, as the basic furniture of reality rather than treating any one of them as more fundamental than the rest.