  This alternative fits well with the apparent facts about Lucretius’ use of sources. So far as one can tell, the material on physics that he started from and reworked was all taken from the first fifteen books of Epicurus’ 37-book magnum opus, On nature. Thanks to the accidental survival in Lucretius’ book 4 of two alternative programmatic passages for the book, we can work out that what we call book 4 was initially planned to come directly after book 2, a sequence of topics which would have exactly reproduced Epicurus’ own in On nature, and that it was only in a later phase that he reorganised his material so that our book 3 came to intervene. This gives good grounds for the guess that the relatively small number of other demonstrable departures from Epicurus’ original sequence likewise represent the process by which, either while or after completing a first draft, Lucretius set about reorganising the poem’s contents into the six-book structure we possess today. So far as one can tell, his rewriting of books 1–3 was complete, but that of books 4–6 was still under way at the time of his death. (This is all argued at length in Sedley 1998.) Among other signs of incompleteness, the latter three books are very long, and would probably have been cut down to something like the length of books 1–3 in the final revision.