User interface design is a hard task. As human-computer interaction is subject to an often mentioned "theory gap", there exists no agreed best method to design user interfaces for a particular task domain, neither is there yet a sufficient coherent body of knowledge and experimental results from cognitive psychology to predict whether a design will be usable. A design approach suggested by many influential and best-selling texts is to employ user interface metaphors. By making the depiction and behaviour of on-screen model worlds appeal to users' existing knowledge structures through metaphor, the unfamiliar target domain of the computing system should be easier to learn and more comprehensible than if no metaphor were employed. Closer examination of user interface metaphors reveals that they can often create serious usability problems while solving others. The scope of a metaphor is often limited, breakdowns in the system illusion occur due to the pragmatics of implementing model worlds on unpredictable hardware. In addition, the Objectivist philosophical tradition underlying previous accounts of metaphor in HCI has become subject to serious attack, some concluding that metaphor can play no role in understanding.

Working within a non-Objectivist world view, Lakoff (1993) and Johnson (1987) propose a contemporary theory of metaphor where metaphors are structured by, and grounded in, image schemas. These serve to give pattern and meaning to orderings of our perceptions and actions that arise from our being embodied and interacting with a tangible physical world. Within this theory, an utterance is either understood directly, or metaphorically, in terms of a common schema.

The Apple Macintosh trashcan is notorious. Its use as a mechanism for ejecting floppy disks causes users real distress at first, who feel that their files will be deleted. One is required to consider the metaphor as carrying across concepts from the on-screen model world to the underlying software, this is not the traditional view of user interface metaphor, but is one arising from user interface semiotics and the notion of user interface similes. We show from analysing the task the user performs within the model world, and the intended change of system state, in terms of the Lakoff/Johnson theory and through a different choice of domains between which the metaphorical mapping is made, that the difficulties with the trashcan arise from the intended meaning of the action sequence performed by the user is the opposite of the literal meaning. Such an utterance is normally termed ironic. In an environment where all actions in the model world are consistent, a unique instance of irony causes understandable problems. We report on other existing user interface features and show how they can be accounted for using the Lakoff/Johnson theory.

Johnson, M. (1987) The Body in the Mind. University of Chicago Press.

Lakoff, G. (1993) The Contemporary Theory of Metaphor. In A. Ortony (ed.) Metaphor and Thought 2nd Edition, Cambridge University Press.pp202-251.