Unanswerable Questions about Images and Texts

It will be useful to setting up a general, abstract framework in which to discuss these issues. Generally speaking AI systems, and for that matter computer programs of any kind for a particular task, the actual ultimate objective can be formulated as follows. There is a class X of inputs that are "reasonable" problems for Q. There is a class Y of possible outputs. The task defines a relation Q(x, y) meaning "y is a good output [or an acceptable output, or the best possible output] on the task for input x." We assume that for every x X there is at least one y Y such that Q(x, y).