Philosophical monism is the belief that reality is fundamentally one, rather than being made up of multiple parts or aspects. This concept is often associated with Parmenides, a pre-Socratic philosopher who argued that reality is a single, unchanging, and indivisible whole. In his view, the idea of multiple things or beings existing independently of each other is an illusion, and only the unity of reality can be truly known.

Parmenides' monistic philosophy was based on the belief that reality must be consistent and logically coherent, and that anything that is contradictory or changeable cannot be real. He argued that the concept of change, for example, involves the existence of something that comes into being and then ceases to exist, which is impossible in a truly unified reality. Therefore, he concluded that reality must be a single, unchanging entity that is eternal and unalterable.

This monistic view of reality has had a significant influence on Western philosophy and has been adopted and adapted by many later philosophers, including Plato and Aristotle. It is also related to other philosophical concepts, such as pantheism and idealism, which also emphasize the unity and fundamental oneness of reality.