  (2) In addition to offering resistance to motion, Leibniz thought it essential to bodies that they resist mutual collocation – to some degree, they must be “solid” or impenetrable.  In attributing solidity to bodies, Leibniz rejects the Cartesian suggestion that impenetrability simply follows from the property of a body’s being extended. For, Leibniz argues, it could be the case that a body’s solidity is due to a body’s having a certain reluctance – but not an unconquerable one – to share a place with another body, and thus he implies that it is, for example, at least conceivable that two bodies might be forced to overlap in much the same way that we generally imagine bodies and regions of space to coincide (NE, Book II, Ch. iv). Having distinguished solidity from extension, Leibniz goes on to further distinguish solidity from hardness. He suggests that whereas solidity concerns the ability of a body to resist being collocated with another body, hardness concerns the ability of a body to resist changing its shape or structure. Thus a body, according to Leibniz, might be perfectly solid (i.e. impenetrable) and yet not hard, a point he illustrates by noting that “if two bodies were simultaneously inserted into the two open ends of a tube, into which each of them fitted tightly, the matter which was already in the tube, however, fluid [or ‘soft’] it might be, [the matter] would resist just because of its sheer impenetrability [i.e. ‘solidity’]” (NE, Book II, Ch. iv).