What “one substance” means
Monism is not simply the claim that the universe is made of one physical material. It is the broader idea that, at the deepest level, reality belongs to one basic kind. The apparent variety of things—stones, trees, thoughts, and people—must therefore be explained as differences within a single underlying reality, not as evidence of several fundamentally separate kinds of existence.
The word substance can be misleading here. In philosophy, it means something basic enough to exist in its own right, not necessarily a tangible stuff like water or dust.
One starting point, different accounts
Monists disagree about what the fundamental reality is. A materialist version treats matter, or the physical world, as basic; minds and thoughts must then be understood as physical processes or as arising from them. An idealist version takes mind or experience as fundamental and explains the physical world in relation to it. Other approaches describe the underlying reality as neither mental nor physical in the familiar sense.
These are not interchangeable claims. Saying that everything is ultimately one does not settle what that “one” is, or how the many things we encounter emerge from it.
Why the distinction matters
The choice changes how a thinker understands the relationship between mind and body. If mind and matter are fundamentally different kinds of substance, explaining how they interact becomes a central problem. If both belong to one basic reality, the task shifts: explain why they appear so different and how each fits into the whole.
That is monism’s lasting challenge as well as its appeal: it seeks unity without making the world’s visible variety disappear. The question is whether a single foundation can genuinely account for the differences we experience, rather than merely giving them one name.
