Non-dual philosophy is often summarised as the claim that everything is one. That summary flattens a set of positions which their proponents took considerable trouble to distinguish.
The question is what happens to difference
A straightforward monism says there is one substance and apparent variety is arrangement. Non-dual arguments in the Indian traditions are concerned with something else: the status of appearance itself.
If plurality is real, the position is not non-dual. If plurality is simply nothing, ordinary experience becomes unaccountable.
Much of the classical debate sits in that gap, seeking a description of the world as neither finally real nor straightforwardly unreal.
Advaita treats appearance as superimposition
The best-known formulation holds that difference is superimposed on an undivided reality, using the standing example of a rope mistaken for a snake.
The snake is not real, but the error is not nothing either. It has effects, it can be corrected, and its correction leaves the rope unchanged.
On this account the world is not denied so much as reassigned. It is genuinely experienced while not being what it is taken to be.
Qualified positions kept distinction inside unity
Other schools accepted a single reality while insisting that distinctions are real features within it, comparing the relation to that between a body and what animates it.
This preserves devotion, since a worshipper and the object of worship remain genuinely distinct even while not being ultimately separate.
The disagreement with stricter non-dualism is therefore not decorative. It determines whether relationship is a final feature of reality or a provisional one.
Buddhist non-duality proceeds differently
Several Buddhist positions reach non-dual conclusions without positing an underlying single substance at all, arguing instead that things lack independent existence.
The move is from unity to dependence. Nothing stands on its own, so the boundaries we draw are conventional rather than found.
Classical debates between these schools were sharp, since one side was accused of smuggling in a permanent ground and the other of dissolving everything into nothing.
The distinctions matter for practice
What a tradition says about unity shapes what it asks a practitioner to do, whether that is inquiry, devotion, analysis or ethical discipline.
Popular presentations often merge these into a single message about oneness, which loses the reasoning that made each position distinctive.
Reading the source arguments restores the disagreements, and the disagreements are where the philosophical content actually lies.