This is an edited version of a talk I gave at Pinner Philosophy Group on 14 May 2018
Here, in the first part of the 21st century, we all seem to be suffering from a form of cognitive dissonance. On one hand, the philosopher of mind David Papineu has declared, ‘We are all physicalists now’. Only a minority of philosophers – and perhaps no natural scientists – endorse substance dualism: the view that mind and body are distinct substances, or completely different types of stuff. It seems that we all subscribe to a form of monism, holding that there is only one substance or type of substance, just one single reality.
Or do we? Edwin Curely has suggested that Cartesian dualism is ‘educated common sense’ – simply the way that adults in our culture conceive of the world in terms of the mental and the physical.
In 2006, in a tragic turn of events, a British man, John Hogan pushed his two children from a balcony in Crete and then jumped after them, following an argument with his wife. Hogan and one of the two children survived, but his six-year-old son sadly did not. Joannis Nestoros, a professor of psychiatry from the University of Crete, conducted a series of interviews with Hogan (who was later found not-guilty of murder on grounds of insanity). He said, ‘The situation will not reoccur, the only possibility is self-harm and suicide because of the guilt. His body did this, not his brain – he wasn’t himself that night.’
Framing the mind-body problem
On the 16 May 1643, Princess Elisabeth of Bohemia began a lengthy correspondence with Descartes. In her first letter, she raised a problem with his ‘real distinction’ between mind and body, which Descartes held to be different substances:
So I ask you please to tell me how the soul of a human being (it being only a thinking substance) can determine the bodily spirits, in order to bring about voluntary actions. For it seems that all determination of movement happens through the impulsion of the thing moved, by the manner in which it is pushed by that which moves it, or else by the particular qualities and shape of the surfaces of the latter. Physical contact is required for the first two conditions, extension for the third. You entirely exclude the one [extension] from the notion you have of the soul, and the other [physical contact] appears to me incompatible with an immaterial thing.
Descartes’ attempt to explain how such mind-body interaction could take place, eventuated in his Passions of the Soul where he claimed that the mode of interaction could be explained through the operations of the pineal gland, which he designated the ‘principle seat of the soul’.
[T]he machine of the body is so composed that, merely because this gland is moved diversely by the soul or any other cause there may be, it drives the spirits that surround it toward the brain’s pores, which guide [the spirits] through the nerves into the muscles, by means of which it makes them move the members.
Like Elisabeth, Spinoza failed to see how it could be possible that a thinking substance, a mind, could interact with the extended or physical substance, the body. Descartes’ putative solution, which Spinoza called ‘a hypothesis more occult than any occult quality’, cuts little ice:
Again, I should like very much to know how many degrees of motion the mind can give to that pineal gland, and how great a force is required to hold it in suspense. For I do not know whether this gland is driven about more slowly by the mind than by the animal spirits, or more quickly […] And, of course, since there is no common measure between the will and motion, there is also no comparison between the power, or forces, of the mind and those of the body. Consequently, the forces of the body cannot in any way be determined by those of the mind (V Preface).
A monist solution
According to Spinoza’s version of the principle of sufficient reason (PSR), everything must have a cause.
For each thing there must be assigned a cause, or reason, both for its existence and for its nonexistence (IP11D).
It is important to note that the ‘cause’ and ‘reason’ are not distinguished here but conflated as terms which are almost equivalent. Because attributes can only be conceived through themselves, it follows that each mode can only be explained under one attribute (IIP6). Putting this together with the PSR, it follows that each mode will form part of a causal chain of modes of the same attribute.
Spinoza gives very little space to arguing for the parallel relation between modes of extension and modes of thought, boldly asserting that it follows clearly from IA4, the axiom which states:
The knowledge of an effect depends on, and involves, the knowledge of its cause.
However, perhaps we can fill in the gaps. Let us say the relata of an instance of a cause and an effect to be two modes of extension E1 and E2 respectively. Now, according to IIP3, there is an idea in God of everything that follows from his essence. Therefore there must be ideas, modes of thought that correspond to E1 and E2. We can call these T1 and T2 respectively. Given the explanatory dependence asserted in IA4 (‘The knowledge of an effect depends on the knowledge of its cause’), the knowledge of the thing E2, that is to say the idea T2, depends on the knowledge of E1, which is to say the idea T1. So we arrive at a correspondence relation between modes of extension and modes of thought.
As each effect is itself a cause, the chains continue ad infinitum:
… → T1 → T2 → T3 →…
… → E1 → E2 →E3 →…
Of course the relations of cause and effect might involve one cause with several effects and vice versa, creating a network of causal nodes. But what is important is that the network of ideas is isomorphic with the network of things: there are two corresponding systems with a one-to-one relation between modes of extension and modes of thought mapping causal and explanatory chains in one onto causal and explanatory chains in the other. 
Spinoza states this succinctly:
The order and connection of ideas is the same as the order and connection of things (IIP7).
This central proposition expresses a theory often referred to as Spinoza’s parallelism, a term which conjures up the image of two distinct rails, which always follow one another but never meet. In one way this image is helpful, as Spinoza rules out interaction between modes of different attributes as I shall later explain. However there is another equally important sense in which it is misleading. From the premise that there is only one substance, comprehended sometimes under one attribute and at other times under another attribute, Spinoza holds that similarly, ‘a mode of extension and the idea of that mode are one and the same thing, but expressed in two ways’ (IIP7S).
So what we have now is not two entities running on parallel rails. Rather we might think of a single causal/logical chain of interacting bodies/ideas, perhaps more like a monorail, a single process that can be viewed from different sides, in different ways, manifesting itself to us under two different attributes (mind and extension). The view that an object (or ‘formal’ being) is identical to the idea (or ‘objective’ being) that represents it is grounded in the medieval scholastic conception of ideas as having what they called esse intentionale. This is a form of being that objects have in thought, so that an idea and its object have, as Lloyd suggests, ‘a kind of sameness’. Spinoza gives an example of a circle that exists in nature and the idea of the circle (IIP7S). Both are in God or Nature and are identical to each other. The difference only pertains insofar as they are conceived either under the attribute of extension or the attribute of thought.
The identity of mind and body
Spinoza now applies his conception to the relationship between the human mind and the human body. The human mind is a finite idea, and one that must have as its object an existing finite thing (IIP11).
Spinoza’s argument rests on two axioms – claims for which he does not argue, taking them to be self-evident:
We feel that a certain body is affected in many ways. (IIA4)
We neither feel nor perceive any singular things, except bodies and modes of thinking. (IIA5).
Since as embodied human beings we are aware of the ways in which our body is affected (IIA4) and can feel nothing other than the body or the mind itself, the object can be no other than the human body:
The object of the idea constituting the human mind is the body, or a certain mode of extension, which actually exists, and nothing else (IIP13).
A human being (‘man’) is an individual that consists of a complex mode of thought, the human mind, and a complex mode of extension, the human body (IIP13C). However, rather than these being two distinct parts of each individual human, their relation is once again one of identity:
The mind and the body are one and the same individual, which is conceived now under the attribute of thought, now under the attribute of extension (IIP21S).
There remains the vital question of how the causal and explanatory relation between the modes of different attributes is to be understood. The conception of an attribute cannot involve the conception of any other attribute (IP10). It follows that a mode of a given attribute can only have God for its cause, as expressed under the same attribute (IIP6). A causal relation between modes of different attributes would contradict this, requiring one attribute to be conceived through another. This entails what has come to be called Spinoza’s ‘explanatory barrier’ between the attributes:
The body cannot determine the mind to thinking, and the mind cannot determine the body to motion, to rest, or to anything else (if there is anything else) (IIIP2).
In this way, the problem of interaction simply does not arise for Spinoza in the same way as it does for Descartes. All the same, Spinoza is quick to admit that his audience will find this claim counter-intuitive:
They are so firmly persuaded that the body now moves, now is at rest, solely from the mind’s command and that it does a great many things which depend only on the mind’s will and its art of thinking (IIIP2S).
Spinoza is not daunted by the task of overcoming his readers’ firm persuasion. He claims the evidence of our experience supports his arguments. For example, sleepwalkers perform actions when asleep that they would not do when awake, so demonstrating that the body is capable of acting from its own nature alone. So in the face of our Cartesian intuitions, Spinoza insists that it is simply not the case that bodily movements arise from the mind or that the mind animates the body. Even if contemporary science could not yet account for the way the body functions, there is no reason in principle that the movements of the body could not be given a purely physical explanation. As he points out, ‘no one has yet determined what the body can do, that is, experience has not yet taught anyone what the body can do from the laws of Nature alone’ (IIIP2S).
It is key to Spinoza’s project that the laws of Nature should be understood as applying universally, in the human sphere just as they do everywhere else:
Therefore, I shall treat the nature and powers of the affects, and the power of the mind over them, by the same method by which, in the preceding parts, I treated God and the mind, and I shall consider human actions and appetites just as if it were a question of lines, planes, and bodies (IIIPref).
Spinoza aims to address the tensions in Cartesianism by naturalizing human psychology. He sets out to show that the mind, as the idea of the body, is subject natural laws, just like the rest of nature:
The affects, therefore, of hate, anger, envy, and the like, considered in themselves, follow with the same necessity and force of Nature as the other singular things. […] I shall consider human actions and appetites just as if it were a question of lines, planes, and bodies. (III Preface)
Spinoza describes and explains the passions using the psychological language of beliefs and desires. However, it must be remembered that for Spinoza, each of these ideas, is an idea of some thing, and that thing exists in the body (IIP13).
Spinoza defines a passion twice, the first time in the definitions of Part III:
By affect I understand affections of the body by which the body’s power of acting is increased or diminished, aided or restrained, and at the same time, the ideas of these affections.
Therefore, if we can be the adequate cause of any of these affections, I understand by the affect an action; otherwise a passion (IIID3).
There are two key features of the way Spinoza defines a passion. Firstly, it is a change that the individual undergoes, either positively, by becoming more capable (passing to a greater perfection or having a greater power of acting), or negatively, becoming less capable (passing to a lesser perfection or having a lesser power of acting). Secondly, while affects can be both actions and passions, Spinoza understands the passivity of the passions, as determined by the fact that their cause comes, at least in part, from outside the individual. Actions on the other hand have their cause wholly caused within the individual.
This can also be understood under the attribute of thought. As distinct from an action, the passion is caused by both an idea in the mind (which must be an idea of a state or change of the body), and an idea that exists outside it: the idea of an external object:
The actions of the mind arise from adequate ideas alone; the passions depend on inadequate ideas alone (IIIP3).
This conception of a passion in terms of ideas is elaborated in Spinoza’s second definition of a passion, which incorporates both its mental and physical aspects:
An affect which is called a passion of the mind is a confused idea, by which the mind affirms of its body, or of some part of it, a greater or lesser force of existing than before, which, when it is given, determines the mind to think of this rather than that (III General Definition of the Affects).
Spinoza identifies a passion (under the attribute of thought) as a confused or inadequate idea. But an idea in the mind must always be an idea of something in the body, and for Spinoza, the idea’s corresponding object is the increase or decrease in the perfection of the body, and so also, its power of acting.
The Primary Passions and the Conatus
Spinoza lists three primary affects: desire, joy and sadness. Central to his psychology is the concept of conatus, the essential striving of a thing, including the human individual, to persevere in its being (IIIP6). When related to the mind and body together, Spinoza terms this striving “appetite” and he explains desire, the first of the affects, as “Appetite together with consciousness of the appetite” (IIIP9S).
Things can both increase or diminish the body’s power of acting and correspondingly the mind’s power of thinking. This accounts for the other two primary passions. Joy is the passage from a lesser to a greater perfection, and sadness, the passage from a greater to a lesser perfection (IIIP11; III Definitions of the Affects II & III).
Love, hate and anger
Spinoza explains the secondary passions through relations between primary passions and ideas, as can be seen, for example, in his definition of love:
Love is a joy, accompanied by the idea of an external cause (III Definitions of the Affects VI).
For Spinoza, it seems, no physiological description is needed. Each passion can be explained as a species of desire, joy or sadness, defined by its relation to an idea or belief.
According to Spinoza, the mind both strives to persevere in its being and is conscious of this striving (IIIP9). Therefore, when the mind imagines things that diminish the body’s power of action (which might include apprehension through the senses as well as forming a mental image), then it strives to recollect that which excludes the existence of such things. Given his definition of love as a joy (an increase in power) with an imagined external cause, if follows that the lover will wish to continue to imagine or experience the beloved object, and so,
We see, then, that one who loves necessarily strives to have present and preserve the thing he loves (IIIP13S).
Spinoza makes little reference to the physiological aspects of passions, and anger is no exception. Rather he defines it in terms of two other passions: desire, a primary passion, and hate, which is produced in a similar pattern to love, but by the combination of the opposite primary passion (sadness), along with a belief:
Hate is sadness with the accompanying idea of an external cause (IIIP14).
Spinoza takes a view here of the affected (or impassioned) individual as being, in some sense rational. If he hates someone, that is if he has the idea of that person as the cause of his sadness, he will desire and strive to remove that cause (IIIP39). The definition of anger then, takes hate as the cause or motive of a species of desire:
Anger is a desire by which we are spurred, from hate, to do evil to one we hate (III Definitions of the Affects XXXVI).
Spinoza’s quasi-mathematical approach to understanding emotion should not be seen as purely an attempt to create a systematic science of human affects for its own sake. Towards the end of the Ethics, we learn that humans can achieve at least a degree of freedom from negative emotions, not through sheer force of will, but by the power of understanding. It is by virtue of our knowledge we attain power over our passions.
In particular, we need to understand that excessive love towards an object that is impermanent (for example riches and fame) can only lead to bondage to negative passions:
Next, it should be noted that sickness of the mind and misfortunes take their origin especially from too much love toward a thing which is liable to many variations and which we can never fully possess. For no one is disturbed or anxious concerning anything unless he loves it, nor do wrongs, suspicions, and enmities arise except from love for a thing which no one can really fully possess.
From what we have said, we easily conceive what clear and distinct knowledge – and especially that third kind of knowledge, whose foundation is the knowledge of God itself-can accomplish against the affects. Insofar as the affects are passions, if clear and distinct knowledge does not absolutely remove them, at least it brings it about that they constitute the smallest part of the mind. And then it begets a love toward a thing immutable and eternal, which we really fully possess, and which therefore cannot be tainted by any of the vices which are in ordinary love, but can always be greater and greater and occupy the greatest part of the mind and affect it extensively. (VP20S)
Why should any of this matter today? After all, we have much more sophisticated understandings of brain functions, a neuroscience informed by experiments using fMRI scans and are equipped with the means to alter mood with psychotherapeutic drugs. Why do we continue to study the somewhat opaque writings of a seventeenth century lens-grinder, and attempt to makes sense of his difficult and sometimes obscure ideas on the mind-body relation
The brain’s mappings of the body
The neuroscientist Antonio Damasio has throughout his career questioned the ‘Cartesian common sense’ that would separate mind and body, reason and emotion as binary opposites. In this he was inspired by Spinoza’s notion that both the mind and the body were parallel attributes of the very same substances. He writes
Spinoza was serving notice of his opposition to the view of the mind-body problem that prevailed in his time. His dissent stood out in a sea of conformity. More intriguing, however, was his notion that the human mind is the idea of the human body. This raised an arresting possibility. Spinoza might have intuited the principles behind the natural mechanism responsible for the parallel manifestations of mind and body […] I am convinced that mental processes are grounded in the brains’ mappings of the body, collections of neural patterns that portray responses to events that cause emotions and feelings. Nothing could have been more comforting than coming across this statement of Spinoza’s and wondering about its possible meaning.
You too may still be wondering what Spinoza meant by ‘the mind is the idea of the human body.’ But perhaps, like me it is this kind of wonder that drives your engagement with philosophy – the love of and search for wisdom or understanding. I hope that I have been successful, if not in making crystal clear what Spinoza meant in IIP13, at least in sharing some of my own sense of wonder about how one great philosophical minds understood the mind and its identity with the body.
All quotations from Spinoza’s Ethics are from the translation by Edwin M. Curley, published in Penguin Classics. Also available in E.M. Curley (A Spinoza reader: The Ethics and other works. (Princeton, N.J.: Princeton University Press, 1994) and The collected works of Spinoza Vol I (Princeton, N.J. ; Oxford: Princeton University Press, 1988).
 Edwin Curley, Behind the Geometrical Method: A Reading of Spinoza’s Ethics (Princeton: Princeton University Press, 1988)
 ‘Mum tells of balcony death plunge’ Wales Online, 22.01.2008 https://www.walesonline.co.uk/news/wales-news/mum-tells-balcony-death-plunge-2203944 accessed 15.05.18
 Lisa Shapiro (Ed.), The Correspondence between Princess Elisabeth of Bohemia and Rene Descartes (Chicago: University of Chicago, 2007), p.62.
 Rene Descartes, Passions of the Soul, Tran. Stephen H. Voss, (Indianapolis: Hackett,1989), p.38.
 I paraphrase Jonathan Bennett, (‘Spinoza’s Mind-Body Identity Thesis’, Journal of Philosophy, 1981, Vol 78, No 10. pp 573-584). However, this picture still omits all but the two known attributes. Whatever other attributes there are, there must be a similar relationship between their modes and ideas of their modes.
 Genevieve Lloyd, Routledge Philosophy Guidebook to Spinoza and the Ethics (London: Routledge,1996), p.49
 Antonio Damasio, Looking for Spinoza: Joy, Sorrow and the Feeling Brain. London : Heinemann, 2003