From a review on Andrew Janiak's new collection of Sir Isaac Newton's philosophical writings:
How is gravity, key to the entire achievement of the Principia, to be understood? Leibniz charged that it was an occult quality, occult in the sense that it purported to explain but did not explain, at least as Leibniz understood that term. To attribute a "dormitive virtue" to a particular substance (to recall Voltaire's later taunt), does not explain how the powder (sic?) acts as its does.
[Newton] had discovered, to his own satisfaction and to that of his followers, a complex form of agency that linked pendulum to moon, to planet, to comet. Admittedly, the manner in which it operated was mysterious. But he had been able to weave a tight mathematical web that made the action of gravity entirely predictable, both as the power to attract and the capacity to be attracted, the same measure applying to each.
Leibniz still had one further cogent objection to raise. When Newton spoke, as he often did, of the sun attracting or being attracted by a planet, the only sort of agency left open (it seemed) was that of action at a distance. And this was, by general agreement among philosophers from Aristotle's time onwards, simply inadmissible, as Newton himself indeed felt forced to concede. So the appeal to attraction was worse than mysterious, it could not even in principle succeed.
The only possibility left was some kind of non-mechanical agency, taking the term 'mechanical' in the contact-action sense demanded by the "mechanical" philosophy of the day.
Newton had, in effect, pioneered a new form of explanation, dynamic explanation, with the notion of force as its anchor.
So, if I have this straight historically, forces could not accurately have been described as mechanical in the contact-action sense, therefore the notion of the mechanical had to be expanded to accomodate the spirit of the day that demanded that all things be, in the contact-action sense, rational.