Philosophy before Plato There is a wall standing between us and the world of antiquity: the period of the decline and fall of the Roman Empire and the rise to dominance of Christianity. Edward Gibbon connected the two phenomena, blaming the former on the latter. He is in significant part right. Remember that in 313 ce the Emperor Constantine gave Christianity legal status and protection by the Edict of Milan, and not long afterwards, in 380 ce, the Emperor Theodosius i decreed by the Edict of Thessalonica that Christianity was to be the official religion of the Empire, outlawing others. The change brought rapid results. From the fourth century of the Common Era (ce, formerly cited as ad) onwards a vast amount of the literature and material culture of antiquity was lost, a great deal of it purposefully destroyed. Christian zealots smashed statues and temples, defaced paintings and burned ''pagan'' books, in an orgy of effacement of previous culture that lasted for several centuries. It has been estimated that as much as 90 per cent of the literature of antiquity perished in the onslaught.

The Christians took the fallen stones of temples to build their churches, and over-wrote the manuscripts of the philosophers and poets with their scripture texts. It is hard to comprehend, still less to forgive, the immense loss of literature, philosophy, history and general culture this represented. Moreover, at the time Christianity existed in a number of mutually hostile and competing versions, and the effort - eventually successful - to achieve a degree of consensus on a ''right'' version required treating the others as heresies and aberrations requiring suppression, including violent suppression. In its assault on the past Christianity had help from others with a similar lack of interest in high classical civilization: Huns, Goths, Visigoths and others - the ''barbarians'' - whose migrations and invasions into the ever-weakening Roman Empire hastened its collapse. The shrinking of mental and cultural life was both a cause and an effect of diminishing education; fewer books were written and published, prohibitions were imposed on what could be read and discussed, and the predictable consequences of such circumstances followed in the form of increasing ignorance and narrowness. Christianity congratulates itself on the fact that the preservation of fragments of classical literature which managed to scrape through this period of appalling destruction was the achievement of monks, in later centuries, copying some of the manuscripts that survived; and although this was a merely partial, belated and inadequate response to the wanton zealotry of the earlier faithful, one must be grateful even for that. As one would expect, only those texts regarded as most significant and outstanding, by individuals themselves thus regarded, managed to survive - and even so, much of the work of some of the greatest figures perished. Only think: Aristophanes was one of a large number of playwrights in fifth- and fourth-century bce (''Before the Common Era'') Athens.

From quotations and allusions we know the names of about 170 other comic playwrights and 1,483 titles of their plays. All are lost; just eleven out of more than forty of Aristophanes'' own works survive. We have only seven plays by the tragedian Aeschylus out of seventy whose titles we know. Imagine if, of the thirty-six plays printed in the First Folio of Shakespeare''s works (we know of at least one lost play, Cardenio, said to have been co-written with John Fletcher), only four were still extant. If we knew the titles of the other thirty-two, what a mighty speculation they would prompt. Imagine if our remoter descendants had just four of Shakespeare''s plays, no Cervantes or Goethe but only their names and reputations, a fragment or two of Schiller, no Jane Austen or George Eliot but again just admiring mention of them, a few quotations in others'' works from Marx, one leg from Michelangelo''s David, one copy of a copy of a Poussin painting, a single poem by Baudelaire, just a few lines of Keats, and so on - scraps and remnants, and not always from the best of their time; this is how things in fact stand with regard to classical and Hellenic antiquity. (And consider: by the accidents and ravages of history the future might indeed have little more to offer its inhabitants than this.) It is an irony perhaps that it was people associated with another oriental religion - Islam - which, a couple of centuries later, also irrupted into the classical world (or rather, into what was by then the carcass of the classical world), who saved some of that carcass''s legacy from oblivion.

As these thoughts tell us, what we know of Plato''s predecessors in philosophy - they are conventionally known as ''Presocratic philosophers'' even though some of them were contemporaries of Socrates - has come to us in shreds and patches. There are two kinds of sources for our knowledge of them: fragments, which are quotations from them in the writings of later commentators, and testimonia, which are reports, paraphrases or summaries given by later writers. The scholarly task of identifying and collating this evidence is known as ''doxography''. The term ''doxographer'' is also applied to those individuals in ancient times who preserved scraps of the Presocratics'' writings or views by quoting or reporting them. Plato and Aristotle both summarized and quoted Presocratic thinkers - sometimes inaccurately, which well illustrates how careful doxography has to be, given that even these giants could get it wrong. Aristotle is indeed a major source of our knowledge of the Presocratics, because he discussed them often and had three of his students, Eudemus, Meno and Theophrastus, write treatises on various of them. Meno concentrated on their medical writings, while Eudemus wrote about their mathematics and astronomy. Only a few traces of the resulting books survive, as quotations and summaries in the work of yet later writers.

Theophrastus discussed the Presocratics'' theories of perception in his On Sensation and their science in his Tenets of Natural Philosophy. A few sections of the first book survive; only the title of the latter remains. Aristotle and his students were writing about thinkers some of whom lived two hundred years before their time. The next important source is Cicero, writing two hundred years after Aristotle''s time, in the first century bce. Thus already the thread was growing longer and thinner - the thread of memory and transmission of sources (manuscript copies following earlier manuscript copies, with mistakes creeping in). Cicero was a serious student of philosophy who sought to inform his Roman contemporaries about Greek thought. But by his time the first age of philosophical genius had passed, and in the centuries that followed other causes of inaccuracy entered the picture, not least polemics - as in the writings of Clement of Alexandria in the second century ce, whose comparisons between Christian thought and Greek philosophy were not designed to favour the latter. Nevertheless he quotes some of the Presocratics, adding to the doxographical store.

The second century ce in fact offers a fairly rich harvest for doxography. The sceptic philosopher Sextus Empiricus quoted extensively from the Presocratics on knowledge and perception, while Plutarch''s Moralia quotes them on a wider range of topics. An anonymous work of the same period called the Placita (''Opinions'') does the same. This book was originally thought to be by Plutarch, so for convenience its unknown author is called ''pseudo-Plutarch''. Later that century Alexander of Aphrodisias quoted a number of Presocratics in his commentary on Aristotle. In the early third century ce Bishop Hippolytus of Rome wrote a Refutation of All Heresies arguing that Christian heresies arose from Greek philosophy, in the process quoting extensively from the Greek philosophical tradition in order to refute it, thus paradoxically preserving the views he sought to demolish. One of the most useful sources for the history of Greek philosophy is The Lives of the Philosophers by Diogenes Laertius, written in the third century ce. It is an informative and entertaining work, though again not always accurate.

It also sometimes, perhaps indeed too often, relies on legend and hearsay, which tempers its value; but nevertheless its value is great. In addition to summaries of biographies and views it gives a bibliography of philosophical works, demonstrating yet again how much has been lost. There was an earlier text, of course lost, on which the Placita drew, which later served as a source for the ''Selections on Natural Philosophy'' of John Stobaeus in the fifth century ce. That earlier text is attributed to Aetius, who lived around 100 ce, and who is thought to have himself used Theophrastus'' book. Another important fifth-century source is Proclus, one of the last heads of the academy Plato had founded nine centuries before. Plato''s Academy (the ''School of Athens'') was closed by the Emperor Justinian in 529 ce, along with a general ban on the teaching of philosophy because it conflicted with Christianity. A very important doxographical source, for all that it dates from a thousand years after the beginning of Presocratic philosophy, is the writings of Simplicius in the sixth century ce. In his commentary on Aristotle''s Physics Book i he quotes a number of the more important Presocratics, in some cases thus serving as the only source of information we have about their views.

Significantly, he says that his reason for quoting so extensively from one of them, namely Parmenides, giving more of the text than was necessary for his ar.