Parallelism

Historical Origins and Core Idea

In philosophy, parallelism designates a family of views about the systematic correspondence between different domains—most notably between mind and body, or thought and extension—such that events in these domains proceed in strict coordination without standing in direct causal relations to one another. Instead of one domain affecting the other, both unfold in parallel, according to a shared order, structure, or ground.

Historically, the term emerges in early modern metaphysics as part of attempts to resolve the mind–body problem raised by Cartesian dualism. If mind and body are distinct substances, how can they interact? Parallelist responses argue that:

• There is a one‑to‑one or law‑like correlation between mental and physical events, and
• This correlation is not due to direct causal influence from one side to the other.

Different versions explain this coordination in different ways: by appeal to a single underlying substance (Spinoza), by a pre‑established harmony ordained by God (Leibniz), or by non‑reductive but law‑governed psychophysical alignment (19th‑century parallelism in psychology and philosophy of mind).

Parallelism thus contrasts with:

• Interactionism, which claims mutual causal influence between mind and body
• Occasionalism, which claims God intervenes at each instance of apparent interaction
• Reductive physicalism or idealism, which collapses one domain into the other

Instead, parallelism preserves both domains while denying cross‑domain causation.

Spinoza and Attribute Parallelism

Baruch Spinoza is often seen as the originator of a sophisticated form of metaphysical parallelism. His system in the Ethics posits one infinite substance (God or Nature) with infinitely many attributes, of which humans know primarily thought and extension.

Spinoza’s doctrine of the parallelism of attributes can be summarized as follows:

• Thought and extension are two ways of conceiving the same underlying reality.
• For every mode (modification) of extension, there is a corresponding mode of thought, and vice versa.
• The order and connection of ideas is the same as the order and connection of things (bodies).

This is often captured by the slogan: “ordo et connexio idearum idem est ac ordo et connexio rerum” (“the order and connection of ideas is the same as the order and connection of things”).

On this view:

• What we call a bodily state and the corresponding idea are not two interacting entities but two aspects or expressions of one and the same state of the single substance under different attributes.
• There is no causal traffic between thought and extension; all causation is within an attribute (ideas cause ideas, bodies affect bodies).

Spinoza’s parallelism is thus non‑interactionist, not because the mental is insulated from the physical, but because they are ultimately the same reality expressed under different conceptual perspectives. Proponents interpret this as resolving the Cartesian interaction problem by rejecting substance dualism; critics argue that it renders mental causation merely derivative or re-descriptive.

Leibniz and Pre‑Established Harmony

Gottfried Wilhelm Leibniz develops a distinct, though related, form of parallelism in his doctrine of pre‑established harmony. In Leibniz’s monadology, the world is composed of simple, non‑extended substances called monads, each expressing the entire universe from its own point of view.

In this framework:

• There is no genuine physical causation among monads; instead, each monad unfolds its states according to its own internal principle.
• What appears as causal interaction between body and mind (or between bodies) is an illusion, produced by the perfect correlation of their inner developments.

Leibniz speaks of a “pre‑established harmony” instituted by God at creation, such that:

• Mental states in a soul‑monad and physical states in the body’s monads run in perfect synchrony, like two clocks set to the same time.
• There is psychophysical parallelism: your decision to raise your arm and your arm’s rising occur together in a law‑like way, but not because one literally causes the other.

Leibniz’s parallelism differs from Spinoza’s in its pluralism and theism:

• Reality consists of many monads, not one substance.
• The coordination of parallel series is explained by a creative act of God, not by identity of attributes in a single substance.

Supporters find this model elegant in preserving both mental and physical regularity without interaction; detractors question its explanatory power, sometimes charging it with being a “cosmic coincidence” story elevated by divine fiat.

Psychophysical Parallelism and Contemporary Debates

In the 19th and early 20th centuries, psychophysical parallelism became influential in German philosophy and early scientific psychology (e.g., in the work associated with Fechner and Wundt). It was often framed as a methodological or metaphysical thesis:

• For every mental event there exists a correlated brain event.
• These two orders—psychic and physical—are strictly coordinated but do not causally interact.
• Scientific psychology could study the mental order while physiology studied the physical one, without reduction of one to the other.

This form of parallelism sometimes functioned as a middle position between:

• Materialist reductionism, which identifies mental processes with brain processes, and
• Substantive dualism, which posits an independent mental substance interacting with matter.

In contemporary philosophy of mind, explicit endorsement of parallelism is relatively rare, but the concept remains important as:

• A historical reference point in debates on mental causation and supervenience.
• A conceptual tool for framing views that emphasize law‑like correlations between mental and physical while remaining non‑reductive.

Parallels are drawn between classical parallelism and modern views such as:

• Epiphenomenalism, which allows physical‑to‑mental correlation but denies mental‑to‑physical causation (though epiphenomenalism typically retains one‑way physical causation).
• Some readings of non‑reductive physicalism, where mental properties systematically co‑vary with physical properties without being reducible to them, raising questions about whether the mental genuinely causes anything over and above the physical base.

Critics of parallelism argue that it:

• Threatens the efficacy of the mental: if mental states never cause bodily actions, our intuitive self‑understanding is undermined.
• Risks being explanatorily idle, simply redescribing correlations rather than explaining them.

Defenders respond that:

• Parallelism can avoid the interaction problem of dualism and the reduction problem of strict physicalism.
• It can be seen as a way of respecting the integrity of both scientific and first‑person descriptions without forcing them into a single causal narrative.

As a result, while classical parallelism in its theistic or monadological forms is primarily of historical interest, its central intuition—systematic, non‑causal correspondence between different levels or aspects of reality—continues to inform discussions in metaphysics, philosophy of mind, and the interpretation of psychophysical laws.