When all is said and done, which model of the mind–body relationship emerges from an examination of Spinoza's conception of the affects? Exploring how the power of acting operates in the affects has made it possible to show how the psychophysical union varies, and reconciles a similarity of order and principle with the mind and body's autonomy of expression. The first point therefore is to stop searching for any interaction, influence or reciprocal causality between the mind and the body, and to think of them only in terms of correspondence and correlation.

From this perspective, the American neurologist Antonio Damasio was right in maintaining that in Spinoza ‘in a strict sense, the mind did not cause the body and the body did not cause the mind’. However, Damasio is not always worthy of his model and sometimes lacks rigour, since he continues to talk about the mind arising from the body and going from the neural to the mental level, which Spinoza could not have accepted. It is true that it is naturally very difficult to do without the categories of cause and reciprocal action when conceptualising the mind–body relationship, and it is tempting to bring them back in another guise. In this respect, meditating on Spinoza's model is an excellent safeguard against attempts to establish a relationship on the basis of an interaction or influence model, which would always retain its ‘occult’ qualities.

Although Spinoza's model excludes interaction in favour of correlation, it cannot be reduced for all that to an interplay of parallels between mind and body that express the same thing under two different attributes. This is the second point to make from studying the affects. Spinoza's model of the psychophysical union is not based on parallelism but on equivalence. This equivalence is not uniformity, since it cannot be reduced to echoing the same thing and to mechanical repetition.

The Three Categories of Affect: Mental, Physical and Psychophysical

Establishing a typology of affects according to their constituting principle and the relationship they have with mind and body appears, at first glance, to be a pointless and arduous undertaking. It is pointless because Spinoza does not engage in this type of inventorying exercise. Above all, he draws three main distinctions with regard to affects: between actions and passions, between primitive affects and combinations of them, and between good and bad, according to their effect on the power of acting. It is arduous because it would be impossible to review all the physical, mental and psychophysical affects, given that listing them would be endless since there can be infinite combinations of emotions within natura naturata.

Spinoza, however, does not reject the principle of differentiating the affects according to their relationship with mind and body, since he uses it to justify his omission of the definitions of cheerfulness, pleasure, melancholy and pain. He does not trouble himself either with the false problem of a comprehensive list, because his aim is not so much a complete as a sufficient inventory of the affects. Without naming them one by one, he nevertheless covers all the affects since, despite their limitless number and specific characters, he knows that they all boil down to kinds of desire, joy or sadness. That is why, following Spinoza's example, it is not a question of drawing a complete picture but, rather, of identifying whether an affect is physical, mental or psychophysical according to the dominant or equivalent role mind and body play in its formation. To that end, it is a good idea to follow Spinoza's classification system and use the three affects he calls ‘primitive, or primary’ (primitivos, seu primarios), to understand how all the other affects are generated and their relationship to the mind or the body. On this basis, it is possible first to distinguish the affects expressly related to both.

The Essence of the Problem

For Spinoza, the human, consisting of a mind and a body, is not, however, a double being composed of two distinct entities. The mind and body must be thought of as a single unit, not as the conjunction of two extended and thinking substances. In fact, for him ‘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.’ Thus Spinoza rules out the possibility of dualism while establishing the possibility of a twofold mental and physical approach to human reality.

But if mind and body constitute one and the same being expressed in two ways, to understand human nature clearly and distinctly we need to understand how these two modes of conception interrelate and come together. The mind (mens), according to Spinoza, is neither a substance nor a receptacle nor a faculty. It is the idea of the body. Therefore, the term mens describes nothing other than the perception or, more precisely, the conception that humans have of their own body – and by extension of the external world – through the various states affecting them. An idea, in fact, is defined as a concept the mind forms because it is a thinking thing. By openly preferring the term ‘conception’ to ‘perception’, Spinoza emphasises the active, dynamic nature of the power of thought operating in the production of ideas. Consequently, the mind is a way of conceptualising the body or forming an idea of it that is more or less adequate, depending on whether the affections modifying it are of a clear or confused nature.

By identifying the mind with the idea of the body, Spinoza provides guidance on how to conceptualise their relationship. He invites the reader to think of their union according to the model of the idea and its object. After establishing in Proposition 13 in Part II of the Ethics that ‘the object of the idea constituting the human Mind is the Body’, he concludes in the Scholium that ‘from these [propositions] we understand not only that the human Mind is united to the Body, but also what should be understood by the union of Mind and Body’.

The Problem of the Originality of Spinoza's Conception

Despite appearances, the author of the Ethics is probably less innovative than he seems, since he was not the first to analyse the affects as joint manifestations of the mind and the body, or to have introduced a mixed discourse. In his Letter to Elisabeth dated 21 May 1643, Descartes had written that the passions come under the union of the soul and the body, and are explained by this ‘primitive notion’. The passions therefore do not depend solely on the soul or solely on the body, but on soul and body together. Indeed, they have both a physical and a mental aspect, since the soul's emotions generally involve an action of the body. The Cartesian definition therefore keeps mind and body together and involves both at once. Passion is a physical reality as regards its cause and a mental one as regards its effects. That is why it is not explained by the primitive notion of thought or by the primitive notion of extension, but only on the basis of the union. Yet the union implies that we conceive of soul and body as one thing. This is what emerges from Descartes’ Letter to Elisabeth of 28 June 1643, in which he maintains that ‘to conceive of the union between two things is to conceive of them as one single thing’. After meditating on the factors that prove the distinction between soul and body, he invites his correspondent ‘to represent to herself the notion of the union that each always experiences within himself without philosophising, in knowing that he is a single person who has together a body and a thought.’ The authorship of the third discourse of ontological identity therefore belongs to Descartes, who tries to go beyond the dualism of substances to conceptualise a human being in his unity. Yet, among the derived notions that allow us to re-examine this union are the voluntary actions of the soul on the body and the passions.

The concept of passion plays a key role in the Cartesian system, since it demonstrates this union operating under the influence of the institution of nature, and allows us to shed light on its incomprehensible essence.

The Reasons for the Comparison

Although Spinoza breaks completely with Descartes in order to found a new science of the affects in the Preface to Ethics III, the fact remains that the author of the Passions of the Soul left a mark on the former's youthful works. Indeed, in the Short Treatise, the analysis of the nature of the affects still embraces a Cartesian model on at least two levels.

First, the nature of the causality that governs the relationship between the soul and the body remains in part Cartesian. Admittedly, Spinoza distances himself from Descartes by refusing to accept that the body is the principal cause of the passions of the soul. The proximate cause of the affects (lijdingen) in the soul is knowledge – in other words, a mode of thinking. Affects can only be experienced by a being who conceives and who is constituted by modes of thinking. Thus love, desire, and all of the affects in general stem from three different modes of knowledge by which humans understand themselves: opinion (waan), which is based on a conception through hearsay and experience; true belief (geloof), which involves apprehending things and their necessity through reason without direct vision; and clear knowledge (klaare kennisse), which does not come from being convinced by reasons but from the awareness and enjoyment of the thing itself. All affects and passions, such as hate and aversion, that are contrary to right reason come from opinion. All affects or passions that are good desires, such as legitimate self-satisfaction and humility, which encompass knowing one's perfection according to its true worth, arise from belief. Finally, clear knowledge produces pure, true love, namely the love of God.

Although all affects are the product of a mode of knowledge, Spinoza nevertheless continues to accept the idea of reciprocal action of the soul on the body and vice versa. In the Short Treatise, Spinoza is therefore partly subject to the very critiques he would later elaborate in Part III of the Ethics, since he contemplates the possibility of an interaction between the soul and the body and one producing an effect on the other.

The renowned American neurologist Antonio R. Damasio, in his book Looking for Spinoza: Joy, Sorrow and the Feeling Brain (2003), explores what the great Dutch philosopher Baruch Spinoza can teach us about the nature of feelings. Having critiqued Descartes in his previous book Descartes’ Error (1994), Damasio, head of the department of neurology at the University of Iowa Medical Center, positions himself firmly in the Spinozan camp (Spinoza avait raison – ‘Spinoza was right’ – is the title of the French translation of the work) in order to shed light on the psychophysical processes brought into play by feelings. Damasio regards Spinoza as a precursor of modern neurobiology and confesses:

This interest in the Spinozan conception of the mind–body relationship is not an isolated phenomenon. It goes well beyond the current infatuation of American neurologists because it is shared by a number of French researchers, including Jean-Pierre Changeux, the author of Neuronal Man, in which he claims a line of descent from Spinoza in his dialogue with Paul Ricoeur, What Makes Us Think? At the outset, he highlights the kinship between his own approach and that of Spinoza:

Throughout the work, the Spinozan conception of the union of mind and body serves as both paradigm and common ground for a phenomenologist and a neurobiologist to tackle the difficult issue of the relationship between the brain and thought.

The Change in Terminology and the Translation Problem

The innovative nature of Spinoza's theory is first expressed at the level of vocabulary through the substitution of the word ‘affectus’ for ‘emotio’ or ‘passio’ to refer to affective movements in people. In Spinoza, the main concept is no longer emotion, but affect. This change in terminology, however, must not be understood as a sweeping innovation or as a radical rejection. On the one hand, the term affectus already appears in Descartes and refers to passion or emotion in its double psychophysiological dimension. This is what emerges from the Latin text of the Principles of Philosophy, particularly §190 in which Descartes tackles the affects of the soul (animi affectibus) as well as natural appetites, and first equates emotions, passions and affects, then affects and passions. On the other hand, the concept of emotion, although rare, is present in Spinoza, and it is synonymous with affect. Thus, in Part IV of the Ethics, he says that ‘the true knowledge of good and evil arouses disturbances of the mind (animi commotiones)’. Now, since the true knowledge of good and evil is defined as an affect, it is clear that the terms are interrelated and overlapping. Spinoza himself corroborates this conclusion by equating affects and emotions on several occasions, such as in Proposition 2 of Ethics V, where he refers to ‘emotions, or affects’ (animi commotionem, seu affectum); in the Scholium to Proposition 20 of Ethics V, where he maintains that ‘when we compare the affects of one and the same man with each other, and find that he is affected, or moved (affici sive moveri) more by one affect than by another’; and in Political Treatise, I, 4, where he asserts that he has

Although there is no reason to pit affectus and commotio against each other, the broad use of the first term rather than the second serves to draw our attention to the originality of Spinoza's definition.