Aristotle died in the autumn of 322 BC. He was sixty-two and at the height of his powers: a scholar whose scientific explorations were as wide-ranging as his philosophical speculations were profound; a teacher who enchanted and inspired the brightest youth of Greece; a public figure who lived a turbulent life in a turbulent world. He bestrode antiquity like an intellectual colossus. No man before him had contributed so much to learning. No man after him might aspire to rival his achievements.

Of Aristotle's character and personality little is known. He came from a rich family. He was allegedly a dandy, wearing rings on his fingers and cutting his hair fashionably short. He suffered from poor digestion, and is said to have been spindle-shanked. He was a good speaker, lucid in his lectures, persuasive in conversation; and he had a mordant wit. His enemies, who were numerous, accused him of arrogance. His will, which has survived, is a generous document. His philosophical writings are impersonal; but they suggest that he prized both friendship and self-sufficiency, and that, while conscious of his place in an honourable tradition, he was properly proud of his own attainments. As a man, he was, perhaps, admirable rather than amiable.

He did not think himself singular in possessing such a desire, even if he pursued his object with a singular devotion; for he affirmed that 'all men by nature desire to know', and he claimed that each one of us is, most properly speaking, to be identified with his mind, so that life -- a fully human life -- is 'the activity of the mind'. In an early work, the Protrepticus or Exhortation to Philosophy, Aristotle announced that 'the acquisition of wisdom is pleasant; all men feel at home in philosophy and wish to spend time on it, leaving all other things aside'. The word 'philosophy' designates, etymologically, the love of wisdom; and a philosopher, in Aristotle's book, is not a cloistered academic engaged in remote and abstract speculation -- he is someone who searches for 'knowledge of things human and divine'. In one of his later works, the Nicomachean Ethics, Aristotle argues that 'happiness' -- that state of mind in which men realize themselves and flourish best -- consists in a life of intellectual activity. Is not such a life too godlike for mere mortals to sustain? No; for 'we must not listen to those who urge us to think human thoughts since we are human, and mortal thoughts since we are mortal; rather, we should as far as possible immortalize ourselves and do all we can to live by the finest element in us -- for if in bulk it is small, in power and worth it is far greater than anything else'.

A man's proper aim is to immortalize himself, to imitate the gods; for in doing so he becomes most fully a man and most fully himself. Such self-realization requires him to act on that desire for knowledge which as a man he naturally possesses. Aristotle's recipe for 'happiness' may be thought severe or restricted, and he was surely optimistic in ascribing to the generality of mankind his own passionate desire for learning. But his recipe came from the heart: he counsels us to live our lives as he himself tried to live his own.

One of Aristotle's ancient biographers remarks that 'he wrote a large number of books which I have thought it appropriate to list because of the man's excellence in every field': there follows a list of some 150 items, which, taken together and published in the modern style, would amount to perhaps fifty substantial volumes of print. And the list does not include all of Aristotle's writings -- indeed, it fails to mention two of the works, the Metaphysics and the Nicomachean Ethics, for which he is today most renowned. It is a vast output; yet it is more remarkable for its scope and variety than for its quantity. The catalogue of his titles includes On Justice, On the Poets, On Wealth, On the Soul, On Pleasure, On the Sciences, On Species and Genus, Deductions, Definitions, Lectures on Political Theory (in eight books). The Art of Rhetoric, On the Pythagoreans, On Animals (in nine books), Dissections (in seven books), On Plants, On Motion, On Astronomy, Homeric Problems (in six books), On Magnets, Olympic Victors, Proverbs, On the River Nile. There are works on logic and on language; on the arts; on ethics and politics and law; on constitutional history and on intellectual history; on psychology and physiology; on natural history -- zoology, biology, botany; on chemistry, astronomy, mechanics, mathematics; on the philosophy of science and on the nature of motion and space and time; on metaphysics and the theory of knowledge. Choose a field of research, and Aristotle laboured in it; pick an area of human endeavour, and Aristotle discoursed upon it.

Of all these writings barely one-fifth has survived. But the surviving fraction contains samples of most of his studies, and although the major part of his life's work is lost, we may still form a rounded idea of his activities.

Most of the surviving writings were perhaps never intended to be read; for it seems likely that the treatises which we possess were made up from Aristotle's lecture notes. The notes were made for his own use and not for public dissemination. They were no doubt tinkered with over a period of years. Moreover, although some of the treatises owe their structure to Aristotle himself, others were plainly put together by later editors -- the Nicomachean Ethics is evidently not a unitary work, the Metaphysics is plainly a set of essays rather than a continuous treatise. In the light of this, it will hardly be a surprise to find that the style of Aristotle's works is often rugged. Plato's dialogues are finished literary artefacts, the subtleties of their thought matched by the tricks of their language. Aristotle's writings for the most part are terse. His arguments are concise. There are abrupt transitions, inelegant repetitions, obscure allusions. Paragraphs of continuous exposition are set among staccato jottings. The language is spare and sinewy. If the treatises are unpolished, that is in part because Aristotle had felt no need and no urge to take down the beeswax. But only in part; for Aristotle had reflected on the appropriate style for scientific writing and he favoured simplicity. 'In every form of instruction there is some small need to pay attention to language; for it makes a difference with regard to making things clear whether we speak in this or that way. But it does not make much of a difference: all these things are show and directed at the hearer -- which is why no one teaches geometry in this way.' Aristotle could write finely -- his style was praised by ancient critics who read works of his which we cannot -- and some parts of the surviving items are done with power and even with panache. But fine words butter no parsnips, and fine language yields no scientific profit.

Aristotle has a vigour which is the more attractive the better it is known; and the treatises, which have none of the camouflage of Plato's dialogues, reveal their author's thoughts -- or at least appear to do so -- in a direct and stark fashion. It is easy to imagine that you can overhear Aristotle talking to himself.

Above all, Aristotle is tough. A good way of reading him is this: Take up a treatise, think of it as a set of lecture notes, and imagine that you now have to lecture from them. You must expand and illustrate the argument, and you must make the transitions clear; you will probably decide to relegate certain paragraphs to footnotes, or reserve them for another time and another lecture; and if you have any talent at all as a lecturer, you will find that the jokes add themselves. Let it be admitted that Aristotle can be not only tough but also vexing. Whatever does he mean here? How on earth is this conclusion supposed to follow from those premises? Why this sudden barrage of technical terms? One ancient critic claimed that 'he surrounds the difficulty of his subject with the obscurity of his language, and thus avoids refutation -producing darkness, like a squid, in order to make himself hard to capture'. Every reader will, from time to time, think of Aristotle as a squid. But the moments of vexation are outnumbered by the moments of elation. Aristotle's treatises offer a peculiar challenge to their readers; and once you have taken up the challenge, you would not have the treatises in any other form.