Many Things, None of Them New
Parmenides had shown that nothing can come from nothing. The pluralists accepted that entirely, and then asked whether change could be saved by giving up unity instead.
The strategy is the same in both: keep a plurality of things that are themselves ungenerated and imperishable, and explain all apparent change as rearrangement. Nothing new comes into being; things only mix and separate. Parmenides is satisfied, and so is observation.
Empedocles proposed four roots — earth, water, air and fire — each eternal and unchanging, plus two cosmic forces, Love which combines and Strife which separates. Six principles in total, and the cosmos passes through cycles as one force dominates and then the other. A bone or a leaf is a particular ratio of the four roots, so qualitative difference reduces to proportion, and nothing has to be created.
He also gave the first recognisable theory of natural selection: creatures arose with limbs combined at random, and only the viable combinations survived. The mechanism is wrong in detail and the structure, variation plus differential survival, is startlingly modern.
Anaxagoras went further and stranger. He held that "in everything there is a portion of everything": matter is infinitely divisible, and every portion however small contains some of every kind of stuff. Bread can become flesh and bone because bread already contains flesh and bone, in small proportion. A thing appears to be what predominates in it, not what is exclusively in it.
Ordering the mixture requires something outside it, and Anaxagoras introduced nous, mind, as the first cause of motion: unmixed, infinite, and the only thing that is purely itself. This is the first appearance in Western philosophy of an ordering intelligence distinct from matter. Socrates records his disappointment on reading Anaxagoras: nous is invoked to start the cosmic rotation and then never used to explain why anything is arranged well.
Common pitfall: treating Empedocles' four elements as a naive guess. They were a considered response to a specific argument, the point is not the number four but the strategy: unchanging plural principles plus mixing, which satisfies Parmenides while saving appearances.