Version: v1, Published online: 2016
Retrieved December 12, 2018, from https://www.rep.routledge.com/articles/thematic/mind-and-body-in-early-modern-philosophy/v-1
Another theme that emerged repeatedly in modern discussions of the nature of the soul was that of simplicity, of the soul being a simple thing. Simplicity itself was often connected to immateriality, because material things – in particular the material things that seem the mostly likely candidates to be material minds, such as brains – are complex systems. This connection of simplicity and immateriality is sometimes associated with a famous passage in Leibniz’s ‘Monadology’ in which he uses the example of a mill.
Moreover, we must confess that the perception, and what depends on it, is inexplicable in terms of mechanical reasons, that is, through shapes and motions. If we imagine that there is a machine whose structure makes it think, sense, and have perceptions, we could conceive it enlarged, keeping the same proportions, so that we could enter into it, as one enters into a mill. Assuming that, when inspecting its interior, we will only find parts that push one another, and we will never find anything to explain a perception. And so, we should seek perception in the simple substance and not in the composite or in the machine.
(Leibniz  1989: 215)
Leibniz concludes here that nothing complex or mechanical could ever explain perception or thought. Rather, he asserts, perception is done by simple substances. But why can the composite not explain perception? One possibility is that Leibniz is offering an argument from inconceivability: an argument that starts from the thought that we cannot conceive or understand how a mechanical device, large or small, could give rise to thought. This inconceivability is used as evidence that a thinking material thing is impossible and thus that thinking things must be immaterial simple substances.
Christian Wolff is often thought of as a follower of Leibniz. Indeed, one reads references to the ‘Leibnizian-Wolffian philosophy’, which might suggest that they held a unified view. As always in these matters, the details are rather more complicated. But we definitely can find Wolff echoing Leibnizian themes about the soul being a simple thinking thing, quite unlike material things.
Because a body cannot think according to its essence and its nature, and because neither a body nor matter can be given the power to think, the soul cannot be anything corporeal and cannot consist of matter. And since it is clear … that thought cannot be attributed to a composite thing, the soul must be a simple thing.
(Wolff  2009: 48)
Immanuel Kant later discussed these issues of simplicity and immateriality in his Critique of Pure Reason (Kant  1999), in particular in the ‘Second paralogism of simplicity’. Here Kant discusses an argument, related to those of Leibniz and Wolff, that the soul cannot be a complex thing, such as a material system, but must instead be simple:
For because the representations that are divided among different beings … never constitute a whole thought … the thought can never inhere in a composite as such. Thus it is possible only in one substance, which is not an aggregate of many, and hence it is absolutely simple.
(Kant  1999: A352)
Moreover, Kant notes that the reason why people want to show the simplicity of the soul is to show that it is not corporeal (Kant  1999). However, he argues, one cannot do what some have hoped to do here. While he agrees that there is a sort of simplicity of consciousness – my thoughts are somehow all collected together and belong to me, one thinker – he also argues that ‘the simplicity of consciousness is thus no acquaintance with the simple nature of our subject, insofar as this subject is supposed thereby to be distinguished from matter as a composite being’ (Kant  1999: A360). The details of his objections are interwoven with his own terminology and approach, but there is a more general question that we can surely ask, separate from the details of his views: even if our experience as thinking things involves a certain (subjective) simplicity, why should we take that to show the (objective, metaphysical) simplicity of the thinking thing itself?
Duncan, Stewart. Simplicity. Mind and body in early modern philosophy, 2016, doi:10.4324/9780415249126-V043-1. Routledge Encyclopedia of Philosophy, Taylor and Francis, https://www.rep.routledge.com/articles/thematic/mind-and-body-in-early-modern-philosophy/v-1/sections/simplicity.
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