Routledge History of Philosophy Volume IV

374 LEIBNIZ: TRUTH, KNOWLEDGE AND METAPHYSICS
philosophy, that the mind is a naturally immortal, immaterial substance. But
these very differences suggest a way of viewing Leibniz’s project, at least with
regard to the mind-body problem; he is seeking to do justice to some of
Spinoza’s key ideas within the framework of traditional Christian theology.
There is indeed much that is Spinozistic in Leibniz’s psychology. Consider,
for instance, how Leibniz applies his concept of expression to the relationship
between mind and body. According to Leibniz, the mind does not interact with
the body, but it expresses it in the technical sense of the term which he explained
for Arnauld’s benefit. Indeed, Leibniz tells Arnauld that the mind expresses its
own body better than it expresses anything else in the universe. 75 In response to
Arnauld’s query Leibniz explains that he does not mean by this that our mind has
clearer thoughts of, say, the activity of its lymphatic glands than of the satellites
of Jupiter; he means rather that given a complete knowledge of my mental states
a supermind would find it easier to read off truths about my physical states than
about the celestial bodies. 76 As Spinoza wrote, ‘the ideas that we have of
external bodies indicate the constitution of our own body more than the nature of
external bodies’. 77 Leibniz might have stopped at this point, but in fact he goes
further; he claims that the mind expresses its body by perceiving it, perception
being a species of expression; indeed the mind perceives everything that happens
in its body. 78 Here again Leibniz seems to be following in Spinoza’s footsteps,
for Spinoza had similarly written that ‘whatever happens in the object [i.e. the
body] of the idea constituting the human mind is bound to be perceived by the
human mind’. 79 But whereas Spinoza does little to dispel the mystery
surrounding this claim, Leibniz offers a body of theory which plugs the gaps in
Spinoza’s account. This is the famous doctrine of unconscious perceptions.
Here it is helpful to recall Leibniz’s hierarchical arrangement of monads. All
monads perceive, but they differ vastly in terms of the quality of their
perceptions. Human minds or spirits are distinguished not only by reason but also
by ‘apperception’ which means consciousness or perhaps even selfconsciousness.
But though Leibniz holds that human minds are set apart from
lower monads by their capacity for (self)-conscious awareness, he further
believes that they also have unconscious or little perceptions (petites
perceptions); such perceptions are little because they are low in intensity. Not
merely do large stretches of our mental life consist wholly in little perceptions,
but even conscious mental states are composed of such perceptions. The doctrine
of unconscious perceptions is perhaps Leibniz’s principal innovation in
psychology, and it is of course profoundly anti-Cartesian in its implications. For
Descartes subscribes to the view that the mind is transparent to itself; he is
explicit that there is nothing in the mind of which we are not conscious. 80
In the New Essays on Human Understanding, his reply to Locke, Leibniz
remarks that there are ‘thousands of indications’ in favour of unconscious
perceptions. 81 Obviously there is an element of hyperbole in this claim, but even
so, Leibniz certainly has a battery of arguments for his doctrine. Some of these
arguments are based on a priori principles such as the identity of indiscernibles