The phrase "sensory order" should be easy enough to grasp. I like to pose it in distinction to William James' idea that the signals detected by our sensory organs constitute a "blooming, buzzing confusion" before they are processed by the brain. (Gertrude Stein studied with William James when she was a student at Radcliffe College. Thomson was a proud Harvard man; but James only shows up in his autobiography for having said, "I am against greatness and bigness in all their forms.") The business of mind, so to speak, is to bring order to all of that confusion; and the primary idea that Hayek contributed to hypothesizing about this problem was that mind has a capacity for what Gerald Edelman would, decades later (but also aware of Hayek's book), call "perceptual categorization." Simply put, order is established when elements of the nervous system (including the brain) detect similarities among sensory signals and express those similarities through the creation of categories. Those categories then provide the first level of processing as new sensory impressions are encountered.
The significance of this insight is that we process new sensory experiences by trying to relate them to past ones. (There is, of course, the question of how the pump gets primed in the first place. Most of Edelman's Neural Darwinism is about answering that question.) This brings up to what Thomson one wrote about how to listen to a new composition for the first time:
Consequently the listener must ask himself what such music most resembles among familiar music of the past and among what he knows of contemporary work, if he is to follow it at all. Such a resemblance may be one of contradiction.Basically, he is arguing that listening to something new in the immediate present should be guided by last listening experiences. Furthermore, that guidance is not necessary driven by a "same-as" relationship but may just as readily result from a "different-than" one. I would thus suggest that, in his own way, Thomson, too, addressed the question of "sensory order," even if he did so within his own particularly specialized domain of sensory experiences.