European philosophers of the seventeenth and eighteenth centuries proposed a wide range of views about the nature of the mind and its relation to the body.
René Descartes (1596–1650) argued that there are two distinct parts to human beings, mind and body, which are substances of radically different kinds, and either of which could exist without the other – although, in fact, in living humans, they are always connected together. He argued for this dualism in several works, including his Meditations on First Philosophy (Descartes [1641] 1984). Elsewhere, in his Discourse on the Method (Descartes [1637] 1985), he argued – based on empirical observation of the difference between humans and other animals – that reason is unique to humans. Indeed, Descartes thought that, because non-human animals do not have an incorporeal mind, they do not even really have sensations.
Though Descartes’s views have been very influential, they attracted critics from the outset. For example, Thomas Hobbes (1588–1679) argued against Descartes that the thinking mind is corporeal, Pierre Gassendi (1592–1655) objected to Descartes’s method of investigating the mind, and Princess Elisabeth of Bohemia (1618–80) objected to his explanation – or lack of explanation – of how the incorporeal mind and the corporeal body are related to one another.
Questions about that relationship continued to divide philosophers in the generation after Descartes. In different ways, occasionalists such as Nicolas Malebranche (1638–1715) and the anti-occasionalist Gottfried Wilhelm Leibniz (1646–1716) both denied that distinct created substances (such as the Cartesian mind and body) could really have a causal influence on each other. Benedict Spinoza (1632–77) denied this too, in his own way, while also arguing for a claim that sounds like a form of materialism: that mind and body are the very same thing ‘expressed in two ways’ (Spinoza [1677] 1988: II, prop. 7, scholium). Other philosophers, such as Hobbes, were more straightforwardly materialists, arguing that the structure and movements of various corporeal systems gave rise to thought. Margaret Cavendish (1623–73), meanwhile, was a materialist who argued that matter itself was fundamentally and irreducibly thinking.
Hobbesian materialism seeks to explain the mind in terms of the body. Idealism, on the other hand, seeks to explain the body in terms of the mind. This is the view that what there are, fundamentally, are incorporeal thinking things and their states (such as thoughts, ideas and perceptions). Material stuff somehow depends on these more basic things. Leibniz proposed this view at some points, as did George Berkeley (1685–1753).
Descartes’s views about animals’ lack of minds also continued to attract attention. A wide range of philosophers thought he must have gone wrong here. This debate has complex connections to others. For example, if you believe that animals can think, but you also believe that thought requires an incorporeal soul, what should you say about animals’ incorporeal souls? What happens to them when an animal dies and their body decays?
Dualist, anti-materialist views were sometimes connected to the notion of simplicity. The idea was that the soul was a simple, indivisible thing, unlike corporeal things such as human brains. Various philosophers thought that they could prove that the soul had to be simple and thus that it could not be corporeal. Leibniz (again), Christian Wolff (1679–1754) and Immanuel Kant (1724–1804) discussed such issues.
Natural Science and Supernatural Thought Experiments