A Definition of Monism
Metaphysical monism is an ancient problem which still continues to this day, at least for some. A definition of monism can be framed quite succinctly; monism states that there is just one kind of thing that exists in the universe, everything is thus reducible to this one thing.
The earliest form of this problem was in ancient Greece. The Greeks had a scientific belief that the world was made up of earth, fire, air and water. What they attempted to understand was whether these four constituents of the universe were ultimate, or was there something more fundamental that underpinned or gave rise to them. They were asking, “Is the world made up of earth, fire, air and water or is the world made up of just one thing that can appear as earth, fire, air and water.”
From our modern post scientific perspective such a view can seem rather primitive. We know for example that the four primitive substances of the ancient Greeks are all reducible to molecules and atoms. We can continue the reduction to protons and neutrons and still further to quarks, or at least to quarks and electrons. The problem has been solved then, or at least the problem as the Greeks saw it has been solved. The debate concerning monism is still alive for some, though in a different format.