The discovery [of universal gravitation] was Newton's, and no informed person seriously questions it.Oddly, though, Westfall's own presentation doesn't appear to provide much support for such a strong statement. Let me explain.
When Hooke wrote to Newton in 1679, he referred to his (Hooke's) system of the world that he had published in 1674, and asked for Newton's opinion of his hypothesis that orbital motions are compounded of a tangential movement and an attraction toward the center. This letter seems to have been an important impetus in reviving Newton's interest in the motion of the planets.
Hooke's 1674 work contained a remarkable paragraph:
This depends on three Suppositions. First, That all Coelestial Bodies whatsoever, have an attraction or gravitating power towards their own Centers, whereby they attract not only their own parts, and keep them from flying from them, as we may observe the earth to do,but that they do also attract all other Coelestial Bodies that are within the sphere of their activity ... The second supposition is this, That all bodies whatsoever that are put into a direct and simple motion, will so continue to move forward in a streight line, till they are by some other effectual powers deflected and bent into a Motion, describing a Circle, Ellipsis, or some other more compounded Curve Line. The third supposition is, That these attractive powers are so much the more powerful in operating, by how much the nearer the body wrought upon is to their own Centers.
Westfall makes a couple of points about this passage. First, he claims that Hooke "did not truly hold a concept of universal gravitation, although it is obvious that he was beginning to break through the limitations of earlier ideas of particular gravities specific to each planet." Even if Westfall is right about this not yet being a thoroughgoing concept of universal gravitation, it is still a remarkable passage, because the comet incident proves that as late as 1680 Newton was still not thinking in terms of universal gravitation.
But Westfall goes on to say that "the most remarkable aspect" of the passage is that "For the first time, it correctly defined the dynamic elements of orbital motion." In fact, it proclaims both "Newton's First Law of Motion": "all bodies whatsoever that are put into a direct and simple motion, will so continue to move forward in a streight line" and, in some form, "Newton's Second Law of Motion": "till they are by some other effectual powers deflected."
It's hard for me as a physicist to read this, and Westfall's acknowledgment that Hooke had it first, and not think that Hooke deserved rather more credit than he has gotten for setting Newton down the right path. It is not until after this time that Newton begins to talk of a centripetal (rather than centrifugal) force. And it is only after this time that he began to ask his astronomer friend, Flamsteed, whether the motion of Jupiter sped up as it approached Saturn and slowed down as it passed beyond (as it would if Jupiter were affected by Saturn's gravity as well as the Sun's). And it is only after this time, in the Principia itself, that he finally applied the same laws of gravitation and orbital motion to comets that he developed for planets (reversing himself on Flamsteed's comet theory).
On some points I find Westfall convincing. Hooke's own complaint missed the mark: he claimed that Newton had learned of the inverse-square law from him, but Westfall demonstrates that Newton had already considered an inverse-square law of some sort back around 1666 or so. And it's clear that, whatever grasp of gravitation and laws of motion Hooke had, he didn't have the mathematical tools necessary to prove, e.g., that the inverse-square law results in elliptical orbits. Newton, with his (still unpublished) calculus techniques, could plunge right in and solve all sorts of mechanics problems, once he correctly identified the key laws.
So it seems to me that Newton owed a lot more to Hooke, both for the concept of universal gravitation (even if Hooke hadn't completely grasped it himself) and for the "dynamic elements" of the laws of motion, than he ever acknowledged.
Of course, I'm probably wrong about this, since I'm not informed and Westfall is. I guess I'll have to read Westfall's other book, Force in Newton's physics, when I'm done with this bio, and see if I can figure out what he means.