 The issues are, however, far more explicit in the entry on  Descartes’ physics;  and since the form of his theory is different, the ‘kinds of motion’ in question are quite different – as they change with all the different theories that we discuss. For Descartes argued in his 1644 Principles of Philosophy (see Book II) that the essence of matter is extension (i.e., size and shape), because any other attribute of bodies could be imagined away without imagining away matter itself. But he also held that extension constitutes the nature of space, and hence he concluded that space and matter are one and the same thing. An immediate consequence of the identification is the impossibility of the vacuum; if every region of space is a region of matter, then there can be no space without matter. Thus Descartes’ universe is ‘hydrodynamical’ – completely full of mobile matter of different sized pieces in motion, rather like a bucket full of water and lumps of ice of different sizes, which has been stirred around. Since fundamentally the pieces of matter are nothing but extension, the universe is in fact nothing but a system of geometric bodies in motion without any  gaps.[1]