The latest generation of synchrotron sources, so-called third generation sources, are able to produce copious amounts of coherent radiation. However it has become evident that the experimental systems that have been developed are unable to fully utilize the coherent flux. This has led to a perception that coherence is lost while the radiation is transported down the beamline. However it is well established that the degree of coherence must be preserved, or increased, by an experimental system, and so this apparent "decoherence" must have its origin in the nature of the measurement process. In this paper we use phase space methods to present an argument that the loss of useful coherent flux can be attributed to unresolved speckle in the x-ray beam.