Marr’s discussion suggests a functional conception of computation, on which computation is a matter of transforming inputs into appropriate outputs. Frances Egan elaborates the functional conception over a series of articles (1991, 1992, 1999, 2003, 2010, 2014, 2019). Like Marr, she treats computational description as description of input-output relations. She also claims that computational models characterize a purely mathematical function: that is, a mapping from mathematical inputs to mathematical outputs. She illustrates by considering a visual mechanism (called “Visua”) that computes an object’s depth from retinal disparity. She imagines a neurophysiological duplicate (“Twin Visua”) embedded so differently in the physical environment that it does not represent depth. Visua and Twin Visua instantiate perceptual states with different representational properties. Nevertheless, Egan says, vision science treats Visua and Twin Visua as computational duplicates. Visua and Twin Visua compute the same mathematical function, even though the computations have different representational import in the two cases. Egan concludes that computational modeling of the mind yields an “abstract mathematical description” consistent with many alternative possible representational descriptions.  Intentional attribution is just a heuristic gloss upon underlying computational description.