The classical picture of the physical world begins with a simple idea:
Reality is composed of systems.
Each system exists in its own right, possesses its own state, and can in principle be described independently of everything else. Interactions occur between systems, but the systems themselves are taken to be prior to those interactions.
This assumption is so deeply embedded in physical thinking that it rarely appears as an explicit thesis.
Yet modern physics quietly undermines it.
The claim of this essay is direct:
The notion of an independently existing physical system is not supported by the structure of contemporary physics.
1. The Classical Assumption of Separability
In classical mechanics, as developed by Isaac Newton, the world is composed of distinct objects.
Each object:
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occupies its own position in space,
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possesses its own properties,
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and can be described independently of other objects.
Even when systems interact, they remain conceptually separable. The state of a composite system is simply the collection of the states of its parts.
This assumption is known as separability.
It underwrites the idea that the world can be decomposed into independently existing units.
2. Composition in Classical Physics
Because systems are separable, larger systems can be built from smaller ones.
A composite system is fully described by specifying the states of its components.
There is nothing over and above the parts and their interactions.
This compositional picture makes independence seem natural:
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first define the parts,
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then describe how they interact.
The ontology begins with independently defined systems.
3. Quantum Theory and Non-Separability
Quantum mechanics disrupts this picture at a fundamental level.
When two systems interact, their joint state is not generally reducible to independent states of each component. Instead, the systems may become entangled.
This phenomenon was first highlighted by Albert Einstein, Boris Podolsky, and Nathan Rosen in their famous argument about the completeness of quantum mechanics.
In an entangled state:
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the composite system is well defined,
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but the individual subsystems are not independently specifiable.
The state of each part cannot be given without reference to the whole.
4. The Failure of Independent States
This has a striking consequence.
In general, it is not possible to assign a definite quantum state to a subsystem independently of its relations to other systems.
What can be specified is:
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the state of the composite system,
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and the statistical structure of outcomes relative to particular measurement contexts.
But the idea that each subsystem possesses its own complete, intrinsic state breaks down.
The independence of systems is no longer guaranteed.
5. Experimental Confirmation
The non-separability of quantum systems is not merely a theoretical curiosity.
It is supported by experimental results related to quantum correlations, including those associated with the Bell's theorem.
These results show that no model based on independently existing systems with pre-defined local properties can reproduce the predictions of quantum mechanics.
The correlations observed in entangled systems cannot be explained by assuming that each part carries its own independent set of properties.
The behaviour of the parts depends on the structure of the whole.
6. Rethinking What a System Is
If subsystems cannot, in general, be assigned independent states, the concept of a “system” itself must be reconsidered.
In classical physics, a system is something that exists in its own right and can be described independently.
In quantum physics, a system is better understood as something defined within a larger relational structure.
Its properties—and even its state—are not intrinsic, but arise within specific contexts of interaction and measurement.
The boundaries of a system are therefore not absolute.
They are defined relative to the theoretical and experimental framework in which the system is described.
7. The Illusion of Independence
Why, then, does the idea of independent systems persist?
Because in many practical situations, interactions are weak or can be effectively ignored. Under these conditions, systems behave approximately as if they were independent.
Classical separability emerges as a useful approximation.
But an approximation is not an ontology.
The success of treating systems as independent in limited contexts does not justify the claim that they are fundamentally independent.
8. A Relational Picture
Once independence is relinquished, a different picture comes into view.
Instead of a world composed of self-contained systems, we encounter:
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structured wholes,
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within which subsystems are defined relationally,
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and whose properties emerge through interaction.
The basic units of description are no longer independent objects, but relations within a structured network.
Systems do not stand outside these relations.
They are constituted within them.
9. The Consequence
The idea that reality is built from independently existing systems is a legacy of classical physics.
Modern physics does not support it.
Quantum theory shows that:
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systems cannot always be assigned independent states,
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their properties depend on relational context,
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and the structure of the whole cannot be reduced to its parts.
The world is not assembled from independent building blocks.
It is articulated through structured relations.
Final Statement
There are no independent systems.
Once this is recognised, the independence assumption that shaped classical ontology loses its foundation.
And with it, a central pillar of the traditional picture of reality quietly falls away.
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