The dating of the three works will be considered later, but it is clear that the first work, the De Venae Sectione adversus Erasistratum, was composed soon after Galen's first arrival in Rome; it is representative, therefore, of his youthful opinions. There is nothing in this work to suggest that Galen, at that stage of his life, considered using any evacuant remedy other than phlebotomy at the onset of the patient's illness. He agrees with Erasistratus that it is necessary to empty the plethos; the only question is by what means it should be evacuated. ‘I have always thought that once evacuation has been agreed on, the easiest and most prompt remedy is to open a vein, since in this way the inflammatory matters, and these alone, are evacuated as quickly as possible.’ All patients with plethos, he says, are most quickly and effectively treated by opening the veins; although Galen mentions alternative remedies used by Erasistratus, there is no indication that he uses any of them himself in these circumstances. Not only the physicians of the rationalist sect, but also the empiricists, all use venesection; even Asclepiades, who condemned all the dogmas of his predecessors and described the Hippocratic methods as an exercise in death. It is in use in most diseases and the most acute, and Hippocrates himself, ‘whom we regard as the leader of all the distinguished men in the profession, and the other men of old clearly did use it’. The ancients, in Galen's opinion, thought it nothing less than the most effective of all remedies.

At the beginning of his first work on venesection Galen expresses his surprise that Erasistratus, a man so punctilious in matters of detail, should have mentioned venesection only once in all his works, and at that only in passing; for, says Galen, it is a remedy of such strength and efficacy that it was regarded by the ancients as nothing less than the most effective of all. It is made clear later that the ancients to whom he refers are, in fact, Erasistratus' predecessors in the profession, including Hippocrates; Galen frequently mentions Hippocrates' use of the remedy in this work, observing that he employed it in most diseases and in the most acute, and that even Asclepiades, who rejected almost all the other methods of Hippocrates, nevertheless still made use of venesection. In another work Galen maintains that venesection has been dealt with very completely by Hippocrates. He is not alone in this opinion. Littré observes: ‘When we enquire which remedies, among the many that were used, are most frequently mentioned as having been applied, we find that bloodletting and the evacuants – emetics and, in particular, purgatives – play the principal role in the therapy of the Hippocratic physicians, and hence of Hippocrates himself.’ Adams expresses a similar opinion in his commentary on Paulus Aegineta. ‘We have had occasion frequently to remark’, he says, ‘that Hippocrates practised venesection freely in various diseases. He has left no treatise, however, expressly on the subject.’

The impression conveyed by both these nineteenth-century writers, and by Galen himself, is that the Hippocratic writers frequently mentioned venesection, and used it freely.

Those who intend to use phlebotomy must consider first of all how many states of the body there are that call for evacuation. The next question is, which of these states require evacuation by phlebotomy; for there are many conditions, some of which need some other sort of evacuation, and certainly not bloodletting. The third problem is to decide which patients can stand the evacuation without harm, since sometimes the patient's condition demands phlebotomy, yet he cannot bear it either because of his stage of life, or the season of the year, or the nature of the country, or because of some disorder of the mouth of the belly. (This is often miscalled the stomach; in the interests of brevity, however, I shall nevertheless use the term from now on in the whole of this work). There are also some who cannot bear phlebotomy because of their general habit of body, even though by reason of their disease they need it very much. If we are to draw such distinctions, we must consider each occasion on its merits, just as we do with any other remedy. Our next question concerns the veins that are to be opened, since extensive research has been undertaken to decide whether it makes no difference which vein one chooses to cut, all being equally helpful in all conditions, or whether, as Hippocrates and the most celebrated physicians thought, it makes a great deal of difference whether one opens this one rather than that.

In Galen's own time, some proponents of venesection believed that it was immaterial which vein was opened, since the same effect could equally well be obtained by using any of them; others, Galen among them, thought that it made a great deal of difference which vessel was used. Hippocrates and the most celebrated physicians, he says, were of this opinion also. This is not an isolated opinion of Galen's; he expresses it in all three of his works on venesection, and he makes further mention of specific sites for bloodletting in the De Sanitate Tuenda, the De Methodo Medendi, Ad Glauconem de Methodo Medendi, and his commentary on the Aphorisms. Now Galen was an expert anatomist, and his view of the structure (as distinct from the function) of the vascular system was, except in insignificant detail, that of the anatomist of today; yet he approved of practices of revulsive phlebotomy which, it would seem, must be quite inconsistent with such a view of vascular anatomy. To try to understand why he did this it will be necessary to summarise not only his own anatomical opinions, but also those of some of his predecessors, particularly those who offered clear accounts of the peripheral vessels.

Since the vessels contain blood, and since blood has been a subject of interest to man from the earliest times, we find some reference to blood-vessels in the earliest Western literature. Homer describes how Antilochus, leaping on Thoon from behind, cut right through the vein that runs straight along the back till it arrives at the neck.

This book, like most jointly written works, has arisen from a shared interest which we have developed together with mutual benefit and pleasure. We are especially grateful for the invention of the word-processor, which has stimulated continuous exchanges and criticisms by making correction pleasing rather than tedious, and which has, we think, resulted in a book which is a product of joint labour throughout, even in parts where one or other of us was originally responsible for more of the ingredients.

We have discussed the modes in several seminars and lectures, and we are grateful to our audiences for their patience and their helpful comments. We hope that the book will encourage further discussion, and that others may succeed in elucidating opaque argumentation where we have failed – and may even discover what Pericles' slave really was doing on the roof-top.

What can we know about the world? How can we think and talk about it? Those two large questions determine two main areas of philosophy. Epistemology discusses questions of cognition: What is knowledge? How much can we know? Of what can we be certain? In what circumstances are our beliefs justified? Logic or philosophy of language is devoted to questions of meaning: What is it for us or our utterances to mean something? How can we refer to things in the external world? By what inferential processes can we legitimately move from one statement to another?

Among Anglo-Saxon philosophers, logic is now often taken to be the fundamental part of philosophy, and questions of meaning are accorded a certain priority: until we understand how we can talk and think about things we can make no progress. Such an attitude to philosophy is young. It traces its origins to the work of the German mathematician and logician, Gottlob Frege, whose active life spanned the period from 1880 to 1925.

Before the age of logic came the age of epistemology. For such thinkers as Descartes and Locke, Hume and Kant, the basic questions of philosophy concerned not language and thought but rather the nature and scope of human understanding. The first task of the philosopher was to determine in what ways and to what extent we can gain knowledge of the world. The triumph of logic has given a distinctive colouring to modern Anglo-Saxon philosophy: in style and approach, in method and argument, it differs noticeably from the philosophy of earlier epochs.