Dual-aspect monism
Explanation
Baruch Spinoza, in his Ethics Demonstrated in Geometrical Order (1677), proposed an elegant alternative to the choice between dualism and materialism: there is a single infinite substance (which he called interchangeably God or Nature), and the mental and the physical are two of its attributes, two distinct ways the same underlying reality manifests itself.
Imagine a sheet of paper: it has a front face and a back face, but they are not two sheets, only two sides of one. Analogously, every real event has a physical aspect (describable in terms of extension, motion, mechanical causation) and a mental aspect (describable in terms of thought, idea, meaning). Neither is more fundamental nor produces the other: they are simply perspectives on the same thing.
This thesis elegantly dissolves the interaction problem that suffocated Cartesian dualism. There is no need to explain how the mind moves the body, because every physical change is ipso facto a mental change, and vice versa. When I decide to lift my arm and the arm rises, these are not two distinct events causing each other mysteriously, but the same event described in two ways.
Spinozism had an immense influence on German Romanticism, on Goethe and Einstein, and on the later tradition of neutral monism defended by Mach, James, Russell and others. They all share the founding intuition: the mental and the physical are too different to reduce one to the other, but too correlated to count as separate substances; they must be two sides of something more basic.
In contemporary philosophy, the dual aspect reappears under names such as Russellian monism and panprotopsychism. The idea is that physics describes structural relations between things but tells us nothing about their intrinsic nature; that very intrinsic nature could be proto-experiential. The mental is not added from outside, but is what matter is on the inside.
The appeal of dual-aspect monism lies in its economy and in its capacity to take seriously both the objectivity of physics and the reality of experience. Its weakness is that the underlying aspect remains in some sense indeterminate: what is that single substance manifesting as mind and matter? Spinoza identified it with God; contemporary philosophers prefer to leave it as a conceptual postulate about the structure of reality.
Strengths
- Elegantly resolves the interaction problem.
- Maintains realism about the mental without reduction or dualism.
- Compatible with modern developments such as Russellian monism.
- Naturalist orientation compatible with science.
Main critiques
- Difficult to specify what the underlying 'single substance' actually is.
- Psychophysical parallelism is postulated rather than explained.
- If every mode has a mental aspect, the door opens to panpsychism with its own difficulties.
- Controversial theological reading: pantheism rejected by religious orthodoxies.