Psychophysical parallelism

Psychophysical parallelism (or parallelism) is the philosophical theory that mental and bodily events occur together, without any causal interaction between them. As such, it affirms the correlation of mental and bodily events, but denies any causal relationship.[1] On this view, mental and bodily phenomena are independent but inseparable– like two sides of a coin. The theory is a third possible alternative of relation between mind and body, between interaction (eg, dualism) and one-sided action (eg, materialism or epiphenomenalism).


A prominent version of parallelism is called occasionalism. Defended by Malebranche, occasionalism agrees that the mind and body are separated but does not agree with Descartes’s explanation of how the two interacted. For Malebranche, God intercedes if there was a need for the mind and body to interact. For example, if the body is injured, God is aware of the injury and makes the body feel pain.[2] Likewise, if a person wants to move their hand, i.e. to grasp an object with their fingers, that want is made aware to God and then God makes the person’s hand move. In reality, the mind and body are not actually in contact with each other, it just seems that way because God is intervening. Occasionalism can be viewed as parallelism with divine intervention so to speak, because if God did not mediate between the mind and body, there would be no interaction between the two.

Monistic parallelism

According to Baruch Spinoza, as explicated in his Ethics, the two attributes of God of which we have cognizance, namely Thought and Extension, are not causally related. Rather, they are two different ways of comprehending one and the same reality. Thus, the human body has a corresponding idea, which is the human mind or soul. Whatever happens in the body always occurs in tandem with contents of the mind. Since everything that exists is a modus of God, Spinoza's concept represents a monist account of parallelism, contrary to Leibniz's pluralist version.


Leibniz, a German philosopher, concluded that the world was made up of an infinite number of life units called monads. A monad (from the Greek monas, meaning “single”) is similar to a living atom, and monads are all active and functioning. As there is naturally a hierarchy in nature, monads vary in degrees of intelligence.[3] Some are more specialized and are more capable of having clearer and more distinctive thoughts opposed to monads that are simpler in structure. Next to God, humans possess the monads that are able to exhibit the highest level of comprehensive thinking. However, humans possess many types of monads, varying from very simple to very complex forms, which explains why the ideas we experience at times differ in clarity.[4] Monads according to Leibniz can never be influenced by anything outside of themselves. Therefore, the only way that they can change, is by internal development, or more specifically, by actualizing their potential. He believed that monads never influence each other; it just seems like they do. Whenever we perceive a monad to be the cause of something, other monads are created in such a way as to seem like they are affecting the other. According to Leibniz, the entire universe was created by God to be in a preestablished harmony, so nothing in the universe actually influences anything else.[5] Looking at psychophysical parallelism in that way, you could imagine the mind and body as two identical clocks. The clocks will always be in agreement because of the preexisting harmony between them, but will never interact. And like the two clocks, no interaction or causation among the monads that make up the mind and body is necessary because they are already synchronized.[6]

However, recent commentary on Leibniz casts doubt on the idea that in the later years of the Monadology [7] Leibniz recognised two distinct categories of mind (psycho) and physical. As indicated in the Stanford Encyclopedia of Philosophy entry on Leibniz's Philosophy of Mind (, and much more extensively in Daniel Garber's 'Body, Substance, Monad' (Garber, D. (2009) Body, Substance, Monad, Oxford, OUP.), Leibniz rejected this sort of Cartesian dualism. In previous years Leibniz had talked of 'corporeal substances' (bodily entities) in addition to the dynamic units that become his Monads. However, the Monadology appears to make it clear, as does much of the late correspondence quoted by Garber, that Leibniz comes to see that Monads are the only atoms of nature and 'body' is merely an illusory appearance of aggregates of monads relating to our confused perceptions. Thus, contemporary Leibniz scholars tend to reject the label of 'psychophysical parallelism' as a misreading.

The origin of the misreading appears to come from the paragraphs 70-81 in Monadology. Paragraph 81 is of note in that it makes it clear that at least some of the preceding discussion is an attempt to explain how there appear to be souls and bodies 'progressing in harmony': 'Ce système fait, que les corps agissent comme si (par impossible) il n'y avait point d'Âmes, et que les Âmes agissent comme s'il n'y avait point de corps, et que tous deux agissent comme si l'un influait sur l'autre.' Leibniz has previously made it clear that everything is Monads and that Monads progress in relation to each other according to God's laws without 'exchanging accidents' in the Scholastic sense - that is to say there is no interaction in the sense of one thing passing something to another. (It may be an over-interpretation to say they do not influence each other since modern physics does not involve 'exchanging accidents' either.) Although Leibniz refers to 'harmony' in paragraph 79 in the context of soul and body (hence the reading of a psychophysical parallelism) the general principle of harmony has already been established for Monads. Moreover, these later paragraphs are intended to deal with a more complex issue which is that although Monads themselves are final causes, their aggregates in the form of body can only be efficient causes. This may seem strange but there are reasons for thinking that it may presage the strange difference between the quantum and classical levels in modern physics (see for a more detailed account of this). Leibniz talks quite extensively of body because in this aggregate form Monads show a different form of causation. Paragraph 79 appears to be intended to indicate that because of the general principle of progression of harmony of Monads, which ultimately underlie body, as well being 'souls', final and efficient causes also appear to harmonise. This might be seen as akin to the correspondence principle in modern physics.

If Leibniz's Monadology is viewed in dualist terms, with both mental and physical ontological categories and a 'psycho-physical parallelism' then it has no counterpart in modern science. On the other hand, if the Monadology is taken as a monistic form of dynamism, which appears to be the predominant view now, then there is no psychophysical parallelism in ontological terms, merely a technical difference in the patterns of dynamics of individuals and aggregates. In this case Leibniz's system may be very much in tune with modern physics in a variety of respects.

See also


  2. Hergenhahn, B. R. An Introduction to the History of Psychology. Australia: Wadsworth Cengage Learning, 2009. Print. p. 185.
  3. Hergenhahn, B. R. An Introduction to the History of Psychology. Australia: Wadsworth Cengage Learning, 2009. Print. p. 187.
  4. Hergenhahn, B. R. An Introduction to the History of Psychology. Australia: Wadsworth Cengage Learning, 2009. Print. p. 187.
  5. Hergenhahn, B. R. An Introduction to the History of Psychology. Australia: Wadsworth Cengage Learning, 2009. Print. p. 188.

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