Monism's oldest argument is almost aggressively simple: Parmenides reasoned that what is cannot come from what is not, so being cannot have arisen and cannot be divided by any real non-being standing between its parts, and must therefore be one, whole and unchanging. Later monists soften the conclusion without abandoning the structure.
Baruch Spinoza's Ethics derives, from definitions and axioms, a single infinite substance he calls God or Nature, of which thought and extension are not two things but two attributes under which the one substance is known; finite things, including human beings, are not separate substances but modes, temporary configurations of that one reality.
Advaita Vedanta arrives at a similar picture from a different direction: Brahman is the sole reality, and the multiplicity of selves and objects is maya, a real experience but not an ultimate one. The Stoics held a physical version of the same claim, treating the cosmos as a single living body pervaded by an active, rational principle.
Critics press one objection against every version: if there is truly only one substance, what grounds the difference between a stone and a person, and how does a changeless whole produce a changing world? Monists answer with the language of modes and levels of truth, which is exactly what their opponents find unsatisfying.