It is important to remember, though, that this definition of place was, in a way, only a convenience. Really, there is no absolute place, since a body can be defined in relation to a whole host of other bodies, not just in relation to contiguous bodies. Similarly, motion is relative, differing based on to which bodies you are comparing the body in question. A man lying at rest on the deck of a ship is moving with respect to the ever-diminishing shoreline. Yet Descartes defines place and motion, strictly speaking, only in reference to contiguous bodies—that is, bodies that share a surface with the body in question. To understand what this means, imagine a raisin bagel that is being twirled around in the air. The raisins in the bagel are in motion when viewed in relation to the surrounding air particles. However, they are not, strictly speaking, in motion on Descartes' view because their position with respect to the bready matter with which they are surrounded remains constant. This is, admittedly, a strange way of defining motion, and it might seem rather arbitrary. Descartes', however, had a strong motivation for making this distinction, as will become clear in Part III.

So far we have gotten through the "contiguous" part of the definition, but what about the "regarded as at rest" part? Remember that Descartes' definition of motion was as a transfer of one body from the vicinity of a group of contiguous bodies regarded as being at rest to the vicinity of another group of bodies. First, why "regarded"? The answer is that we only regard these bodies as at rest, because given Descartes' picture of motion, they cannot really be at rest. Since motion is defined as the transfer of position with reference to another body, when body A moves in reference to body B, body B moves just as much in reference to body A. A body cannot really move, in other words, without its contiguous body also moving. So what is the problem with saying that both A and B move? Why even add in the "regarded as at rest" if neither contiguous body can really be at rest so long as the other is moving? The problem with saying that both A and B move is that it lands Descartes in conflict with Church policy. If whenever A moves, B must move as well, then the earth must necessarily move. Certainly parts of matter with which the earth is contiguous, move with respect to the earth (e.g. the particles of the atmosphere). If these parts move with respect to the earth, then in Descartes' view the earth must move as well. Descartes' solution to this bind is to add in the "regarded as at rest." By adding this clause, Descartes is able to leave himself an escape hatch (however narrow), should the inquisition come calling.