A retrieval system can be studied at three levels: The way in which units of information, and relevant relations between them, are defined in the system. This is the semantic level of subject analysis. The general structural features of the system considered as a network of units of information linked to each other and to documentary items. This may be called structural analysis. The physical mechanisms (hardware) in which the structure is embodied.

p. 1275. - The Structure of Information Retrieval Systems (1959)