5. Since every doubt which has been raised with regard to the universality and accuracy of the law of gravitation, has thus ended in confirming the rule, it is not surprizing that men's minds should have returned with additional force to those views which had at first represented the law as a necessary truth, capable of being established by reason alone. When it had been proved by Newton that gravity is really a _universal_ attribute of matter as far as we can learn, his pupils were not content without maintaining it to be an _essential_ quality. This is the doctrine held by Cotes in the preface to the second edition of the _Principia_ (1712): {275} 'Gravity,' he says, 'is a primary quality of bodies, as extension, mobility, and impenetrability are.' But Newton himself by no means went so far. In his second Letter to Bentley (1693), he says, 'You sometimes speak of gravity as essential and inherent to matter; pray do not ascribe that notion to me. The cause of gravity,' he adds, 'I do not pretend to know, and would take more time to consider of it.' Cotes maintains his opinion by urging, that we learn by _experience_ that all bodies possess gravity, and that we do not learn in any other way that they are extended, moveable, or solid. But we have already seen, that the ideas of space, time, and reaction, on which depend extension, mobility, and solidity, are not results, but conditions, of experience. We cannot conceive a body except as extended; we cannot conceive it to exert mechanical action except with some kind of solidity. But so far as our conceptions of body have hitherto been developed, we find no difficulty in conceiving two bodies which do not attract each other.