  Careful reading of the Principia makes clear that, although unforthcoming about any of the loose ends, Newton was perfectly aware of them all, in one way or another flagging each for the benefit of the highly astute reader. An instructive way to present the history of eighteenth century research in the wake of the Principia is to trace how each of the loose ends became a prominent matter of concern and was then resolved, at least to the point of being removed as in any way a threat to Newton's theory of gravity. This process of addressing the loose ends in Book 3 did not get underway until the 1730s, after Newton had died. During his lifetime the most pressing complaint against the Principia was the absence of a mechanism to account for its action save for action at a distance, which Newton himself regarded as “so great an absurdity, that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into  it.”[56] The absence of a mechanism, however, was not something that Newton himself regarded as a loose-end in the Principia, for he insisted that all the conclusions listed above could be established, and any loose ends in them resolved, through the law of universal gravity alone, independently of the mechanism responsible for it. Over the decades after he died, those engaged in research predicated on his theory of gravity came increasingly to this same view of the question of mechanism.