Galen of Pergamum (129–215 ce), the influential Greek doctor and philosopher, had much to say about the soul and its relationship to the body. In developing his views on psychophysical interaction, Galen used and combined ideas and concepts derived from the philosophical traditions of Platonism, Aristotelianism, and Stoicism. He also took medical views on board, ranging from the earliest fifth-century bce writings of ‘Hippocrates’, the fourth-century doctor Diocles of Carystus, and Hellenistic medical writers such as Herophilus and Erasistratus, to the works of his older medical contemporaries, such as Rufus of Ephesus and the Pneumatist writer Archigenes of Apamea; these were especially relevant when it came to the anatomy and physiology of the soul and the diagnosis and treatment of mental disorder in its somatic aspects.

In the first book of his Tusculans, Cicero gives two arguments that the soul is eternal. Specifically, he concludes that each human soul’s rational part or ‘mind’ (mens) neither came to be nor will perish. I shall argue that in pursuit of this conclusion, Cicero constructs the position that, at least in this life, the human mind does not ‘sense’ itself, but knows about itself only by inference from its ‘sensations’ of other objects. First, the question of the mind’s sensation of itself is comparable to current debates about consciousness, for example to questions relating to the ‘hard problem’ of consciousness. To put it crudely, the ‘hard problem’ is that we suppose that our minds are conscious of themselves immediately, and that what we seem to learn from this immediate self-consciousness is that our minds are so different from the natural world we observe through the senses that it is a puzzle to understand how the two could be as intimately related as they appear to be.

Throughout its long history, Stoic philosophy was subjected to much criticism, from inside the school as well as from outside. This is particularly true when it comes to psychology. Roughly speaking, the Stoic soul is characterised by two main features: first, it is defined as a single and entirely rational substance which has no parts, in particular no irrational parts as is the case in Platonic and Aristotelian theories; this psychological monism is the ground for what I will call the ‘Stoic pledge’, namely that the moral agent is absolutely responsible for all his mental acts: impressions (phantasiai) and assents (sunkatatheseis), and also passions, virtues and vices, sensations, and so on. All of these are in our power, because we always have the ability to avoid them, to have second thoughts about them, and to put them right if they are wrong.

The structure of this essay is roughly as follows. After an introduction in which I rehearse some of the main elements of Stoic physics and psychology, I set out the evidence for the Stoic doctrine that the individual soul is both analogous to the cosmic soul and a part of it, as was held by the early exponents of the school. I argue that the doctrine threatened to land the Stoics in trouble, unless they were ready to qualify it by applying to it certain distinctions. Then I consider the doctrine’s historical origins, discussing several possible antecedents of it, with a focus on a Socratic report in Xenophon and on Plato’s dialogues, and showing that the Stoics went beyond all their predecessors. After this historical excursus I turn to examine some of the arguments advanced by the early Stoics to prove that the cosmos has a soul, including one which compares the relation between the cosmos and individual human beings to that between a father and his offspring. This leads me to analyse in more detail this comparison which occurs also in other contexts and is sometimes problematically connected with the notion that the individual soul is a part of the cosmic soul. One of the texts which are relevant to this issue, a report in Sextus Empiricus, is the object of a separate discussion in the final Appendix.

The relationship of soul to body was one of the earliest and most persistent questions in ancient thought. It emerges in the Homeric poems, where the psuchē is a breath-like stuff that animates the human being until it departs at death for the underworld, leaving the corpse (sōma or nekros) behind. In the Odyssey these souls are found lurking wraith-like in the underworld until they are revitalised by a sacrifice of blood which gives them a temporary power to think and speak again. Among Pythagoreans and others, the soul lives imprisoned in the body until it is liberated at death, only to be reincarnated for a new life in a new body in accordance with its merits. Plato embraces this theory in several of his dialogues, but even though the soul is a relatively autonomous substance it is nevertheless deeply affected by the conditions of the body it inhabits during life and the choices this embodied soul makes. Other early Greek thinkers regarded the soul as little more than the life force animating a body, a special kind of material stuff that accounts for the functions of a living animal but then disperses at death. Democritean atomism embraced this notion of soul, which was also common in the medical tradition.

Much has been written about Epicurus’ psychology. However, despite the enduring popularity of the theme, to the best of my knowledge no contribution has yet specifically been devoted to the reception of Epicurus’ psychology within the Epicurean tradition outside of Lucretius. The present essay therefore begins with some paragraphs of Epicurus’ Letter to Herodotus devoted to the soul, in order to ascertain how Epicurus’ psychology was represented by certain later Epicureans, most notably Demetrius Lacon (second to first century bc) and Diogenes of Oinoanda (second century ad), who lived not just in different ages but in very different places (the former, who never became scholarch, lived between Miletus and the mother school of Athens; the latter, between Oinoanda and Rhodes). In particular, I will set out to determine what the Epicureans made of the so-called bipartition between a rational and a non-rational part of the soul. While this theory is nowhere to be found in the Letter to Herodotus, it is espoused by Lucretius through the distinction between animus (the rational part) and anima (the non-rational part). In order to present as comprehensive a picture of the context as possible, it is best to start with Epicurus, and in particular with a challenging passage in §§ 63–4 of the Letter to Herodotus.

Although Aristotle’s thought became a byword in the early modern period for a rigid and inflexible system impervious to empirical evidence, it seems to have been quite otherwise in the practice of Aristotle’s own school. In reconstructing the psychological theory of the third head of the Lyceum, Strato of Lampsacus, I argue that we can best understand his departures from Aristotelian doctrine by looking to new medical theories in the Hellenistic period, which had implications for our philosophical understanding of the soul’s functioning. Aristotle, who was interested in the natural processes underlying even the most abstract aspects of human functioning, had lampooned the notion that the soul might exist apart from the body: his psychology posited an intimate connection between form and function, and was informed by empirical studies of living organisms. Strato was known even in antiquity both for a revisionist attitude and his focus on natural philosophy. It would thus be unsurprising for him to be interested in new medical research.

For the modern reader, Timaeus 35a counts among the more abstruse passages in Plato’s dialogues. For ancient Platonists, too, it was considered to be obscure, yet also exerted an enduring fascination because it was believed to contain the key to understanding Plato’s concept of the soul. It provides a technical description of the composition and nature of the world soul and can therefore be used better to understand the (rational) human soul, which was held to be structured analogously. Plutarch of Chaeronea is the author of an exegetical work dedicated to this passage. Even though it is the oldest extensive treatment to have come down to us, there is strong evidence, as I hope to show, for an older exegetical tradition, going back to debates in the early Academy. This exegetical tradition may not have been continuous, but there are traces of it even in the Hellenistic era. In this essay I offer a reconstruction of the tradition preceding Plutarch.

Johansen examines the role of internal heat in the theories of nutrition and animal generation in Plato’s Timaeus. There, Plato does not ascribe the status of being besouled to all beings which engage in nutrition, but to beings with perceptive faculties. This raises questions as to the status of nutrition in the explanation of life and besouled beings.

The chapter presents an investigation into the Presocratic background of soul theories, more specifically, the relationship between fire and soul in Empedocles. Trépanier argues that Empedocles fragment B 9 does not give us a positive account of fire as soul, but targets a rival Presocratic account of fire as soul.

Thein provides a detailed analysis of Aristotle’s analogy between “the phusis in the pneuma” and “the element of the stars” in Aristotle’s GA (736b29–737a7). With the help of several passages in GA which discuss the role of pneuma in the generation of living beings, as well as passages of De caelo, where the first body is presented as an animate, self-moving but soulless entity, Thein makes the case that the key to the analogy lies in the shared link between motion and animation, a link which, in these two cases only, does not require the actual presence of the soul.

Betegh explores to what extent the Presocratic philosophers made the motive power of heat topical, and how they tried to provide an explanation of that power. He argues that while the motive power of heat never seems to obtain a principal role in the cosmological theories of the Presocratics, it appears to take a more important role in the explanation of living beings in a number of theories.

Reece discusses Diogenes of Apollonia’s claim that “all existing things are differentiated from the same thing and are the same thing” (DK 64B2, Simp. Phys. 151.31–32), namely air. He examines Diogenes’ principle that causal interaction and change require some sort of uniformity among the relata, and considers the questions which this principle raises.