The abstract properties of this mapping are defined precisely, and its appropriateness and adequacy for the… — David Marr

Vision is the process of discovering from images what is present in the world, and where it is.

[If] we can experimentally isolate a process and show that it can still work well, then it cannot require complex interaction with other parts of vision and can therefore be understood relatively well on its own.

Vision is a process that produces from images of the external world a description that is useful to the viewer and not cluttered with irrelevant information.