When he attacked the old natural philosophy Hobbes complained mainly of its failure to give genuine explanations of natural effects. It left people ignorant of the 'subordinate and secondary causes of natural events' because its explanatory concepts and categories were not cut out for the relevant questions about causes. People who wished to know the causes of heat or gravity or various 'propounded appearances' were asking after efficient causes, not formal and final ones, motions rather than forms or essences. And phenomena or appearances were in question. The old natural philosophy could not fasten in the right way on the appearances of things to sense, because it did not recognize appearances as things in the mind categorically distinct from bodies. So Hobbes's objections go on. We have had an indication of the explanatory categories he rejects. In this chapter we shall see what he puts in their place.