 Despite his evident success in obtaining what we now call the law of universal gravitation, Newton admits that he lacks another kind of knowledge about gravity; this lack of knowledge directly reflects an aspect of his abstract characterization of forces. In the General Scholium, he reminds his readers that gravity is proportional to a body’s quantity of matter (its mass) and reaches across vast distances within our solar system, adding: “I have not as yet been able to deduce from phenomena the reason for these properties of gravity, and I do not feign  hypotheses”.[19]  With this phrase, one of the most famous in all of Newton’s writings, he returned to a key theme of his very first optical paper from forty years earlier, viz. the proper role of hypotheses and of hypothetical reasoning within natural  philosophy.[20]  Some of Newton’s interpreters have regarded this phrase as signaling a strong commitment to the broad doctrine that all hypotheses concerning natural phenomena ought to be avoided in principle. This interpretation is sometimes coupled with the view that some British philosophers in the late seventeenth century regarded Cartesianism as overly reliant on hypotheses in reaching conclusions about phenomena. But this interpretation may be hard to square with Newton’s texts. For instance, in the Scholium to Proposition 96 of Book I of the Principia, Newton discusses hypotheses concerning light rays. Similarly, in query 21 of the Opticks, he proposes that there might be an aether whose differential density accounts for the gravitational force acting between bodies. In light of such examples, one can read the General Scholium’s pronouncement in this way: a philosopher concerned with explaining some feature of nature—such as the fact that gravity is inversely proportional to the square of spatial separation, rather than, say, the cube—may legitimately entertain and propose hypotheses for consideration, but she may not “feign” the hypothesis in the sense of taking it as having been established either through experiment, observation, or some form of reasoning (including mathematical reasoning). Hence Newton thinks that he has established the fact that gravity acts on all material bodies in proportion to their quantity of matter, but he has not established the existence of the aether. What, then, does Newton’s slogan hypotheses non fingo actually rule out? By the time of the General Scholium, Newton was increasingly embroiled in philosophical disputes with Leibniz. After reading the copy of the Principia that Newton had sent him, Leibniz wrote an essay (“Tentamen”) on the causes of planetary motion for the famous journal Acta Eruditorum. In order to account for the motions of the planetary bodies in his Tentamen, published in 1689, Leibniz introduces ex hypothesi the premise that some kind of fluid surrounds, and is contiguous to, the various planetary bodies, and then argues that this fluid must be in motion to account for their  orbits.[21]  Newton may have argued that Leibniz had “feigned” the hypothesis of the vortices. That is, he would have objected to Leibniz’s conclusion that there must be vortices in the solar system (as opposed to the suggestion, for instance, that we try to detect their presence through observations of things like comets). A debate between the two philosophers on this score would bring them to the question of the mechanical philosophy: whereas Newton would object to Leibniz’s reasoning on methodological grounds, Leibniz would reply that Newton’s theory of gravity involves action at a distance, which his vortex hypothesis avoids (see below for more details).