1. A navigation system for a set of materials, comprising: a plurality of attributes characterizing the materials; a plurality of values describing the materials, wherein each of the values has an association with at least one of the attributes and each association defines an attribute-value pair, and wherein some of the attribute-value pairs refine other of the attribute-value pairs; a plurality of navigation states, wherein each navigation state corresponds to a particular expression of attribute-value pairs and to a particular subset of the materials; and an interface, the interface providing a plurality of transitions, each transition providing a direct path between two of the navigation states, wherein each transition represents a change from the expression of attribute-value pairs corresponding to an originating navigation state to the expression of attribute-value pairs corresponding to a destination navigation state, wherein a series of one or more transitions provides a path between any two navigation states, there being more than one path between at least a first of the navigation states and a second of the navigation states.