In approaching metaphor computationally, some authors, e.g., Dedre Gentner (see Falkenhainer et al. 1989), have viewed it as depending on shared properties and relational structure (while allowing for discordant ones), directly attached to the concepts being compared. For example, in comparing an atom to the solar system, we observe a revolves-around relation between electrons and the nucleus on the one hand and between planets and the sun on the other. But others have pointed out that the implicit comparison may hinge on properties reached only indirectly. In this view, finding a metaphor for a concept is a process of moving away from the original concept in a knowledge network in a series of steps, each of which transforms some current characteristic into a related one. This is the process termed “slippage” by Hofstadter et al.  (1995). Others (e.g., Martin 1990, drawing on Lakoff and Johnson 1980) emphasize preexisting knowledge of conventional ways of bridging metaphorically from one concept to another, such as casting a nonliving thing as a living thing.