In his book, A Theory of Semiotics (1976), Eco has argued that the recognition of a relationship of similarity between the sign-vehicle and its signified content is necessarily dependent upon a rule (or rules) of 'similarity". According to this argument, such rules serve to guide us in selecting, from among the many qualities that both a sign- vehicle and its content may possess, only those qualities that are properly to be compared. Thus, for example, the "blue" in the upper portion of a postcard landscape is capable of bringing the notion of 'sky" before our minds only to the extent that we are able to isolate the quality of "blueness' from the other qualities equally possessed by the "postcard sky" and the sky as directly perceived, i.e., qualities such as material density, scale, shape, temperature, texture, weight and two-dimensionality, etc. Obviously, the patch of blue on the postcard "resembles' the sky only in a very restricted sense. Eco's point is that the fact that this simultaneous isolation and correlation of blues appears to be quite automatic should not be allowed to hide from us the fact that it is accomplished on the basis of a network of largely implicit assumptions regarding the nature of postcard photographs and their relationship to objects of direct perception.