Traditionally, motion has been defined extrinsically, as the change in the position of an object, such as a particle, over time. This definition presupposes the existence of an "observer" external to the particle who measures the position of the particle as a function of time. Yet, in the double slit experiment, we have experimental proof that a particle can travel unobserved between a source and a detector. It does so in such a way that we cannot, even in principle, describe its motion by a position changing in time. We propose that the correct interpretation of this experiment is that the traditional extrinsic definition of motion is incomplete. We propose a more general intrinsic definition of motion, in which the role of observer is played by the particle itself, and which presupposes no external observer. We discuss the double slit experiment including the nature of measurement and the particle-wave duality. We show that using the intrinsic point of view allows us to avoid the "weirdness" that tends to crop up when these topics are analysed from the extrinsic point of view.