What is a visual object?

The concept of an 'object' plays a central role in cognitive science, particularly in vision, reasoning and conceptual development - but it has rarely been given a concrete formal definition. Here I argue that visual objects cannot be defined according to simple physical properties but can instead be understood in terms of the hierarchical organization of visual scene interpretations. Within the tree describing such a hierarchical description, certain nodes make natural candidates as the 'joints' between objects, representing division points between parts of the image that cohere internally but do not perceptually group with one another. Thus each subtree hanging from such a node corresponds to a single perceived 'object'. This formal definition accords with several intuitions about the way objects behave.