  Of course, were it not for the Principia, there would be no entry at all for Newton in an Encyclopedia of Philosophy. In science, he would have been known only for the contributions he made to optics, which, while notable, were no more so than those made by Huygens and Grimaldi, neither of whom had much impact on philosophy; and in mathematics, his failure to publish would have relegated his work to not much more than a footnote to the achievements of Leibniz and his school. Regardless of which aspect of Newton's endeavors “Newtonian” might be applied to, the word gained its aura from the Principia. But this adds still a further complication, for the Principia itself was substantially different things to different people. The press-run of the first edition (estimated to be around 300) was too small for it to have been read by all that many individuals. The second edition also appeared in two pirated Amsterdam editions, and hence was much more widely available, as was the third edition and its English (and later French) translation. The Principia, however, is not an easy book to read, so one must still ask, even of those who had access to it, whether they read all or only portions of the book and to what extent they grasped the full complexity of what they read.  The detailed commentary provided in the three volume Jesuit edition (1739–42) made the work less daunting. But even then the vast majority of those invoking the word “Newtonian” were unlikely to have been much more conversant with the Principia itself than those in the first half of the 20th century who invoked ‘relativity’ were likely to have read Einstein's two special relativity papers of 1905 or his general relativity paper of 1916. An important question to ask of any philosophers commenting on Newton is, what primary sources had they read?