If the first of Europe's philosophers whose works survive does not have the same towering dominance as its first poet, Homer, that is not any reflection on Plato's genius. His actual achievement in his own field was as great. It is merely that we know a little more about what went before. Despite this, he, like Homer, presents to us the appearance (albeit a misleading one) of arising out of nothing, and also of a certain primitiveness which his marvellously polished style does not altogether conceal. He has a greater claim than anybody else to be called the founder of philosophy as we know it. But what, exactly, did he found? The answer will depend on who 'we' are; it will be different for Patonists and Latonists, and even that crude division does not do justice to the complexity of Plato's make-up, and of his influence on the subsequent history of philosophy.

Of the two Platos that we distinguished, it is difficult to think that the achievement of Pato was as great as that of Lato. The 'perennial philosophy' is perennial just because it is a very natural expression of human thinking about the mind and about values; it has appeared in many places at many times in different forms, and Plato's mindbody dualism, with its associated belief in the immortality of the soul, and his particular treatment of the objectivity of values, are not markedly different from anybody else's. What is unique in him is the progress from these quasi-religious speculations, which could have remained, as they have in others, vapid and evanescent, towards a much tougher, more precise logical and metaphysical theory, a moral philosophy and a philosophy of language; these were not entirely new, but, through discussion and criticism of them, they engendered the lasting achievements of Aristotle in those fields, and thus shaped the entire future of philosophy.

Let us start with Plato's development of the topic of 'The One and the Many'. We have seen how the early cosmologists sought an explanation of the bewildering variety of things in the world by seeking for them some common ground or reason. The search started with the question, 'What were their origins?'; went on to the question 'What are they all made of?'; but then divided. Natural scientists went on asking this second question in ever subtler forms and have been answering it ever since. But by this time problems had arisen which could not be answered by this method, and which demanded an entirely different sort of inquiry, whether we style it metaphysics or logic. For the puzzles generated by Parmenides could not be solved without asking 'What are they all?' in a quite different sense. This new inquiry, whether we call it conceptual or logical or even linguistic, consists in asking about the meanings of the words we use, or, to put it in a way more congenial to Plato, about the natures (in a quite different sense from the physical) of the things we are talking about. The Many are to be understood, not by seeking their physical constituents, nor even the efficient causes of their motions and changes, but by isolating and understanding the Idea to which we are referring when we use a certain word. This is to know in the deepest sense what it is to be a thing of a certain kind. Plato had grasped the truth that conceptual understanding is different from natural science, and just as important. He had succeeded in distinguishing from each other the four different types of explanation (the four different kinds of 'Why?'-questions and their answers) which were duly classified by Aristotle in his doctrine of the 'four causes'. Of these we have just mentioned three:

1. The material cause, or explanation of the material constitution of a thing;

2. The efficient cause, or cause in the narrower modern sense, which made a thing do what it did;

3. The formal cause, or explanation of its form -- of what it is to be that kind of thing; and he also, as we shall see in a moment, distinguished;

4. The final cause, or explanation of the purpose for which something comes to be as it is.

Plato was more interested in formal and final causes than in the other two kinds, and thought that they would both be understood by getting to know the Idea of the kind of thing in question. This association of the formal and final causes (having its origin in Plato's doctrine about the Good, already discussed) may have been a mis take; but, if so, it was a very momentous one which was taken over by Aristotle and by many philosophers to this day. The notion that what it is to be a thing of a certain kind (its essence) is logically tied to what a thing of that kind ought to be (its purpose) still has its adherents.

To have distinguished the four kinds of explanation would have been achievement enough, but Plato went further. He saw that there was a question about how we could claim to know the answers to the formal and final 'Why?'-questions. We may concede that in his theory of knowledge knowing is treated too much like mental seeing, and the objects of knowledge too much like objects of ordinary vision, being different from them only in being seen by the mind and not the eyes, and in having a perfection and abidingness which the objects of ordinary vision do not have. But nevertheless the Theory of Ideas does represent Plato's way of stating some very important discoveries.

The first of these is that the sort of knowledge we are after both in science and in mathematics and logic is something universal. A causal law or a mathematical or logical theorem, if it holds at all, holds for all similar cases. That moral principles too have to be universal is a feature of them whose importance has to be acknowledged even by those who do not follow Plato in his cognitivism -do not, that is, allow themselves to speak of moral knowledge.

The second is that all these disciplines including morality are capable of being structured into systems in which more general concepts or statements form the grounds of more specific ones. For both Plato and Aristotle this truth was expressed in their doctrine that in order to say what a thing is, we have to say to what genus it belongs, and then to say how it is differentiated from the other kinds of things in that genus. This is summed up in the Platonic method of dialectic, employing 'collection' and 'division' (see p. 44). We must never forget that the word Plato used for his Ideas, ' eidos ', is the same word, and with very much the same meaning, as we translate 'species' when we meet it in Aristotle's logic. Plato's description in the Republic (511) of the way in which the Ideas are subordinated to one another in a hierarchy may sound too crudely physical to us (it is almost as if he were looking with his mind's eye at a lot of quasi-visible onions strung together in a rope); but this was his way of putting the thought that a discipline has to be logically ordered if its propositions are to be connected (the metaphor survives) with each other.

In this and other ways Plato's investigations of the Socratic 'What is . . .?' questions led him a very long way into the disciplines of logic and metaphysics. Aristotle's systematisation of logic -- above all his theory of the syllogism which dominated logic for many centuries -- could never have been achieved without Plato's insights.

Plato also, as we have seen, avoided a trap into which he might easily have fallen, given his assimilation of knowing to mental seeing: that of thinking, as Descartes seems to have thought, that the clarity and distinctness of the vision was a certificate of its correctness. Instead, by recognising the difference between knowledge and right opinion, he was led to demand, as a qualification for knowledge, the ability to give and defend a reason or explanation for the thing known. This explanation normally took the form of a definition (ideally of the type just described). However, the importance of this distinction transcends Plato's particular theory of definition. Whenever anybody, whether in science or mathematics or moral philosophy, makes some statement on the basis of mere intuition, hoping that we will share the intuition and therefore agree with it, he should be disciplined by means of the Socratic-Platonic demand that he 'give an account' of what he has said. Even now too many philosophical frauds are unwilling to face the auditors in this way.

So far we have not, in this chapter, made much of any distinction between on the one hand science and mathematics, and on the other morals and politics. This is in accordance with Plato's practice; he thinks that all are subject to the same disciplines and methods, although in the application of them to this imperfect world rigour may be lost. But those who now wish to make a sharp distinction between evaluative and factual propositions, and thus between the methods appropriate to morality and science, do not have to part company with Plato completely even here. For one of the most remarkable things about him is how, even though he never wavered in his objectivism, and constantly assimilated moral to other kinds of knowledge, he also recognised quite early, following Socrates, the special feature of value judgements which distinguishes them from factual ones, their prescriptivity. This comes out above all in his equation of thinking something good with desiring and therefore being disposed to choose it, and thus in his acceptance, albeit in a modified form, of the links between knowledge and goodness which had led Socrates into paradox.

Nor did the prescriptivity of value judgements die with Plato. It is implicit in Aristotle's statement that the Good is what everything is after; and also in his doctrine known as the 'practical syllogism'. The conclusion of a piece of practical reasoning, he saw, can be an action just because its premisses contain a value judgement which is prescriptive. He insists that practical wisdom, our guide in matters of evaluation and action, is 'epitactic' (meaning 'prescriptive') -- a word he takes over, with the distinction it implies between active prescription and mere passive judgement, from Plato Politicus (260b). The same intimate connection between value judgements and action became important again in the eighteenth century with the work of Hume, who found in it an obstacle to the founding of morality on reason, and of Kant, who thought he had surmounted the obstacle; and it is still important today.

Plato was also the first person in history to attempt a systematic account of the structure of the mind. His account is no doubt crude compared with Aristotle's, let alone with what a satisfactory explanation of 'mental' phenomena would require. And he did not see the necessity for saying precisely what, in more literal terms, the metaphor of 'parts of the mind' really means. All the same, he started a very important and fruitful line of inquiry, and had much more excuse for his crude partition of the mind than some recent thinkers like Freud. Although it is hard to take seriously, as constituents of 'the mind', entities like 'the intellect' and 'the will' (to use modern descendants of Plato's terms), the distinctions which have been made in this kind of way do nevertheless need making.

They need making, above all, in order to emphasise the importance of disciplined thought, if we are to have a satisfactory way of answering any of the more difficult questions that face us. Although we have to allow credit to Plato's predecessors, and especially to the Sophists, for bringing into emphasis the intellectual side of man's nature, we owe to Plato and Socrates more than to anybody else the idea, which has been current ever since, that man will have more success in almost everything he undertakes if he learns to think better.

This brings us to what, I am sure, Plato himself thought of as his most important practical contribution: his educational theory. He believed firmly that there could be a body of knowledge or understanding whose attainment and handing down would make possible the orderly solution of political problems such as had brought Athens and all Greece into chaos. In this he taught the world a valuable lesson. If we could fully understand the problems, which involves understanding first of all the words in terms of which they are posed, and then (even harder) understanding the situations and the people that generate them, we should be on a way to their solution. This, at any rate, is a more hopeful line than attributing them to human wickedness which can never be eradicated. Even the wicked can be coped with if we understand what makes them do what they do. Socrates did not think he had attained this understanding, and even Plato was not all that optimistic; but he saw it as the only way out of the troubles of Greece, and founded an institution, the Academy, which he thought would help towards attaining it.

His bolder plans for political reform are more questionable, and more tentative. If the education of the intellect, preceded by a thorough schooling of the will, is necessary in order to put human society to rights, how can this come about? Plato here took a short cut. If absolute power could come into the hands of good and wise men, would not that do the trick? We have seen how much of good sense can be extracted from this bold suggestion. It is not wholly devoid of merit, but simply ignores the difficulty (indeed the practical impossibility) of finding suitable incumbents, and the further difficulty of reconciling absolute power, however wise its possessor, with the attainment of ends which nearly everybody (and who shall say they are wrong?) will include in their requirements for the good life, above all liberty. When Plato, impressed with the practical difficulties, goes on in the Laws to subject human and fallible rulers to a rigid code, he only makes matters worse. In its final form the Platonic proposal shares many features with the Holy Inquisition.

Nevertheless, Plato's political theory presents the liberal with a challenge which he has to face, and in facing which he will find himself having to answer questions which too many liberals ignore. If some ways of organising society are better than others, in the sense that they do better for the people who live in the society, even on their own reckoning; and if some politicians and others are doing their very best to prevent it being organised, or kept organised, in these better ways, what am I to do about them, if not seek the power to frustrate their malign endeavours? If I think I know how a wise dictator would arrange things, ought I not to try to become a wise dictator? Plato has his answer to this question; what is the answer of the liberals?

Plato did not see his political proposals realised, nor perhaps did he expect to. His only excursion into politics, in Sicily, was a disaster. But a change did come over men's minds as a result of his thought. Greek political morality did not improve, it is true; nor was the Roman much better. But though the practice of politics remained as dirty as before, it is fair to claim that, gradually, through the work of Plato and his successors, the Stoics, Christians and others, ideals of a new and better sort came in the end to be current.

The rhetoric of present-day politics is still mostly nothing but rhetoric; but rhetoric does influence people (even its authors), and cause things to happen which otherwise would not. Our political rhetoric is permeated now by ideals which were simply non-existent in the rhetoric of Plato's day. This can be seen by comparing almost any political speech nowadays with almost any speech reported from the fifth and fourth centuries BC. Politicians do not always do what they commend in their speeches; but sometimes they do, and that has made a difference to the world. Part of this difference we owe to Plato. In the end he made many people see that personal or even national ambition and success are not the most important things in life, and that the good of other people is a worthier aim. For this we can forgive him for being also the father of political paternalism and absolutism.