On the Doctrines of Hippocrates and Plato is Galen's treatise in nine books, written in Greek at Rome between about 162 and 176 CE, arguing that Hippocrates and Plato agree on the parts of the psychē and on the body. Against Chrysippus, who placed the ruling part of the psychē in the heart, Galen holds with Plato that the reasoning part is in the brain, the part that feels anger in the heart and the desiring part in the liver. He argues from dissection, from the logic of demonstration and from Chrysippus' own words, quoted at length, with Posidonius on his side.
Books I to III take up the seat of the ruling part: dissection shows the brain as the source of the nerves and the heart of the arteries, and the Stoic proofs from the voice, from poets and from ordinary speech are answered one by one. Books IV and V turn to the passions, Chrysippus on their cure and Posidonius, with Plato, on irrational movements. Book VI places the desiring part in the liver as the source of the veins; Book VII returns to the brain and to sight. Books VIII and IX compare Plato with Hippocrates on the chymoi and on method.
The work survives in Greek in medieval manuscripts; Book I has lost its beginning.
Galen of Pergamon On the Doctrines of Hippocrates and Plato: summary and contents (PDF)
Galen of Pergamon On the Doctrines of Hippocrates and Plato: summary and contents (Markdown)
Contents
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Αʹ Book I
The first book survives without its beginning: the text opens on the case of a boy whose breastbone rotted away to expose a heart that went on working. Against Chrysippus, Galen argues from dissection that the heart's ventricles hold blood, not pneuma, describes how the carotid arteries reach the brain and form the net-like plexus beneath its membrane, and concludes that the brain is the source of the nerves and the heart of the arteries, leaving the liver and the veins for later.
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Βʹ Book II
Book 2 sorts premises into the scientific and those that are false or beside the point, then examines the Stoic proofs that the ruling part of the psychē sits in the heart, among them the argument from the voice, which Galen answers with a parallel about urine. It ends by showing Diogenes's argument inconclusive: if he holds with Zeno, Cleanthes and Chrysippus that blood nourishes the psychē while its ousia is pneuma, then what nourishes it is not what moves it.
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Γʹ Book III
Chrysippus's book Peri psychēs is the target of Book 3, which Galen says he wrote at friends' urging. He takes Chrysippus's witnesses one by one, the poets, myths, ordinary speech and gesture, and turns them against him: when women say an insult does not go down into them, the phrase speaks of anger, not of reasoning. Throughout, he sets the spirited part of the psychē apart from the reasoning part, as Plato does.
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Δʹ Book IV
Book 4 sets Chrysippus's writings on the passions against Hippocrates and Plato. Galen quotes him at length, especially the book of On Passions on their cure, on passion as an excessive impulse that oversteps logos, and argues that Chrysippus contradicts himself, since an impulse that runs contrary to logos cannot have logos as its cause. The book ends on grief that fades with time though the belief behind it remains, and promises Posidonius's objections for the next book.
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Εʹ Book V
Posidonius is Galen's ally in Book 5. Against Chrysippus, who made the passions judgements, and Zeno, who made them the contractions and swellings of the psychē that follow judgements, Posidonius held with Plato that they are movements of irrational powers, the desiring and the spirited. Galen draws on him for the education of children, with reasoning as the charioteer of desire and anger, and ends by reading Plato's use of Homer to show anger and reasoning as distinct parts.
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Ϛʹ Book VI
Here Galen returns to his first question: do the powers that govern us all start from the heart, as Aristotle and Theophrastus held, or from three sources, as Hippocrates and Plato taught? After defining activity and affection, he argues that the liver is the source of the veins, since anatomists begin teaching the veins from it, and that the appetitive part of the psychē lives there, closing on Homer's Tityos, punished through his liver.
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Ζʹ Book VII
With Stoics and Peripatetics now less confident, Galen notes, Book 7 places the reasoning part in the brain and turns to perception. It explains how pneuma travels to the eyes through the optic nerves and asks how we see, whether objects send something to us or a power goes out from us through the pupil. It closes with Erasistratus, who grasped late in life that the nerves begin in the brain, and Aristotle, who never did.
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Ηʹ Book VIII
Book 8 restates the argument that the ruling part lies where the nerves begin, in the brain, and then compares Plato with Hippocrates on the body. Plato's accounts of bile, phlegm and black bile follow Hippocrates' account of the chymoi, Galen argues, though On the Physis of Man sets it out in better order. The book ends by defending Plato's claim that some drink passes into the lungs, citing animals given coloured water.
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Θʹ Book IX
The last book takes up method: telling the most similar things apart. Galen sets Plato's warning that a deceiver must know likeness and unlikeness beside Hippocrates' criticism, in Regimen in Acute Diseases, of physicians who split each nosos into too many kinds and use too few remedies, and ends on division, distinguishing how dialecticians and philosophers use the term and holding that the reasoning psychē is one, with many powers.