Beneath every statement of this series there lies a mass of careful observation, tested by what John Stuart Mill calls the method of “agreement and difference,” and these valid conclusions are in turn used for the final induction that creation by miracle is unknown. Similarly, in the third book, he demonstrates by use of the same logical process that, since sickness, coma, age, poison, and whatever affects the body, also affect the mind, the mind has actual contact with the body. The standard method of “concomitant variations” is also used frequently as, in the second book, where the argument runs thus: since heavy and light bodies fall more nearly uniformly in thin air than in heavy water they would fall at the same rate of speed in a vacuum. Except in the sixth book, which follows sources closely, Lucretius’ wealth of examples seems to come largely from his own store. In truth, most of Mill’s categories of inductive methods are implicit in Lucretius, for the Epicureans were in his day busily defending their use of induction against the attacks of Stoics. The logical treatise of Philodemus,[11] which of course Mill did not know, seems to have been written very shortly after Lucretius’ death, and it is not at all improbable that Lucretius had heard the lectures of Philodemus before they were finally given to the public. In those lectures the author dwells much on the validity of carefully chosen analogy, for in the field of the unobservable—in evolutionary cosmology, in atomic theory, and in psychology—metaphor and simile have always been and will always be fruitful tools of science. But Philodemus finally insisted on the necessity of basing all inductions on extremely careful observation, of using only essential similarities and pertinent comparisons, and he implied, even if he did not explicitly state it, that every test of “agreement,” “difference,” and “residue” is necessary. Of course the Epicureans fell into the fallacies of incomplete data, as all science based upon inductive methods must, and as beginners they were obviously impatient of delay and over-optimistic; but the correct forms of the inductive processes were all in daily use and if Bacon and Mill had known the treatise of Philodemus, which so well explains the picturesque arguments of Lucretius, they would have shown more respect for the “wisest of the ancients.”