This paragraph is huge, so for the sake of making it easier to read I’m breaking it down (and the paragraph continues beyond the point the excerpt below ends):
What we perceive are never unique properties of individual objects, but always only properties which the objects have in common with other objects. Perception is thus always an interpretation, the placing of something into one or several classes of objects. The characteristic attributes of sensory qualities, or the classes into which different events are placed in the process of perception, are not attributes which are possessed by these events and which are in some manner ‘communicated’ to the mind; they consist entirely in the ‘differentiating’ responses of the organism by which the qualitative classification or order of these events is created; and it is contended that this classification is based on the connexions created in the nervous system by past ‘linkages.’
The qualities which we attribute to the experienced objects are, strictly speaking, not properties of objects at all, but a set of relations by which our nervous system classifies them. To put it differently, all we know about the world is of the nature of theories and all ‘experience’ can do is to change these theories. All sensory perception is necessarily ‘abstract’ in that it always selects certain aspects or features of a given situation. Every sensation, even the ‘purest,’ must therefore be regarded as an interpretation of an event in the light of the past experience of the individual or the species.
— Heinrich Klüver, “Introduction,” in F.A. Hayek, The Sensory Order (University of Chicago Press: Chicago, 1976 ), pp. xviii–xix.