 Once the Principia was published, Newton had a vexed relationship with the mechanical philosophy, an orientation within natural philosophy that is associated with nearly every significant early modern philosopher, including Descartes, Boyle, Huygens, Leibniz, and  Locke.[22]  One of the reasons for this complex relationship can be understood if we consider Newton’s attitude toward forces in an abstract way. His second law indicates that a body moving rectilinearly will continue to do so unless a force is impressed on it. This is not equivalent to claiming that a body moving rectilinearly will continue to do so unless another body impacts upon it. A vis impressa—an impressed force—in Newton’s system is not the same as a body, nor even a quality of a body, as we have seen; but what is more, some impressed forces need not involve contact between bodies at all. For instance, gravity is a kind of centripetal force, and the latter, in turn, is a species of impressed force. Hence a body moving in a straight line will continue to do so until it experiences a gravitational pull, in which case it will deviate from a straight line motion, even if no body impacts upon it. Indeed, the gravitational pull might originate with a mass that is millions of miles away. As we have seen, an impressed force is an action exerted on a body. Hence the gravity exerted on a moving body is an action (the Latin term is actio), which is obviously a causal notion. This is not an empirical claim per se; it is merely a reflection of Newton’s laws, together with his notion of an impressed force, and his further idea that gravity is one kind of impressed force. These elements of the Principia make conceptual room for a causal interaction between two bodies separated by a vast distance, one enabled by Newton’s concept of an impressed force. Aspects of this idea became known in philosophical circles as the problem of action at a distance (Hesse 1961). Many of Newton’s most influential contemporaries objected vigorously to the fact that his philosophy had made room for—if not explicitly defended—the possibility of distant action between material bodies. Leibniz and Huygens in particular rejected this aspect of Newton’s work in the strongest terms, and it remained a point of contention between Newton and Leibniz for the rest of their lives. Both Leibniz and Huygens were convinced that all natural change occurs through contact action, and that any deviation from this basic mechanist principle within natural philosophy would lead to serious difficulties, including the revival of outmoded Aristotelian ideas. By the seventh proposition of Book III of the Principia, as we have seen, Newton reached the following conclusion (1999: 810): “Gravity acts on all bodies universally and is proportional to the quantity of matter in each”. Leibniz eventually accused Newton of regarding gravity as a kind of “occult quality”, that is, as a quality of bodies that is somehow hidden within them and beyond the philosopher’s understanding. They understood Newton to be saying that gravity is a kind of hidden power to attract embedded in material bodies.