I can see part of the front of a person in the audience, and my idea of his shape uses this information to approximate his total shape. Thus I don't expect him to stick out two feet in back even though I can't see that he doesn't. However, my idea of the shape of his back is less definite than that of the parts I can see. The ability to represent and use knowledge about knowledge is often required for intelligent behavior. What airline flights there are to Singapore is recorded in the issue of the International Airline Guide current for the proposed flight day.
Travel agents know how to book airline flights and can compute what they cost.
A program that must co-operate or compete with people or other programs must be able to represent information about their knowledge, beliefs, goals, likes and dislikes, intentions and abilities. Common sense includes much knowledge whose domain overlaps that of the exact sciences but differs from it epistemologically. For example, if I spill the glass of water on the podium, everyone knows that the glass will break and the water will spill. Everyone knows that this will take a fraction of a second and that the water will not splash even ten feet. However, this information is not obtained by using the formula for a falling body or the Navier-Stokes equations governing fluid flow.