Pylyshyn uses constraints to solve many of the problems
associated with the inverse problem in vision. We are sympathetic to
such an approach, and indeed, we think that in many cases constraints
allow tract-able solutions to otherwise insoluble problems. We argue,
however, that Pylyshyn has been too quick to assume that certain
perceptual phenomena can be explained by appealing to constraints
embodied in the visual machinery. For several more complex perceptual
phenomena it is not clear how one proceeds to look for constraints
once simple constraints like rigidity have been abandoned.