next up previous


The Missionaries and Cannibals puzzle, much used in AI, contains more than enough detail to illustrate many of the issues.

``Three missionaries and three cannibals come to a river. A rowboat that seats two is available. If the cannibals ever outnumber the missionaries on either bank of the river, the missionaries will be eaten. How shall they cross the river?''

Obviously the puzzler is expected to devise a strategy of rowing the boat back and forth that gets them all across and avoids the disaster.

Amarel (1971) considered several representations of the problem and discussed criteria whereby the following representation is preferred for purposes of AI, because it leads to the smallest state space that must be explored to find the solution. A state is a triple comprising the numbers of missionaries, cannibals and boats on the starting bank of the river. The initial state is 331, the desired final state is 000, and one solution is given by the sequence (331,220,321,300,311,110,221,020,031,010,021,000).

We are not presently concerned with the heuristics of the problem but rather with the correctness of the reasoning that goes from the English statement of the problem to Amarel's state space representation. A generally intelligent computer program should be able to carry out this reasoning. Of course, there are the well known difficulties in making computers understand English, but suppose the English sentences describing the problem have already been rather directly translated into first order logic. The correctness of Amarel's representation is not an ordinary logical consequence of these sentences for two further reasons.

First, nothing has been stated about the properties of boats or even the fact that rowing across the river doesn't change the numbers of missionaries or cannibals or the capacity of the boat. Indeed it hasn't been stated that situations change as a result of action. These facts follow from common sense knowledge, so let us imagine that common sense knowledge, or at least the relevant part of it, is also expressed in first order logic.

The second reason we can't deduce the propriety of Amarel's representation is deeper. Imagine giving someone the problem, and after he puzzles for a while, he suggests going upstream half a mile and crossing on a bridge. ``What bridge'', you say. ``No bridge is mentioned in the statement of the problem.'' And this dunce replies, ``Well, they don't say there isn't a bridge''. You look at the English and even at the translation of the English into first order logic, and you must admit that ``they don't say'' there is no bridge. So you modify the problem to exclude bridges and pose it again, and the dunce proposes a helicopter, and after you exclude that, he proposes a winged horse or that the others hang onto the outside of the boat while two row.

You now see that while a dunce, he is an inventive dunce. Despairing of getting him to accept the problem in the proper puzzler's spirit, you tell him the solution. To your further annoyance, he attacks your solution on the grounds that the boat might have a leak or lack oars. After you rectify that omission from the statement of the problem, he suggests that a sea monster may swim up the river and may swallow the boat. Again you are frustrated, and you look for a mode of reasoning that will settle his hash once and for all.

In spite of our irritation with the dunce, it would be cheating to put into the statement of the problem that there is no other way to cross the river than using the boat and that nothing can go wrong with the boat. A human doesn't need such an ad hoc narrowing of the problem, and indeed the only watertight way to do it might amount to specifying the Amarel representation in English. Rather we want to avoid the excessive qualification and get the Amarel representation by common sense reasoning as humans ordinarily do.

Circumscription is one candidate for accomplishing this. It will allow us to conjecture that no relevant objects exist in certain categories except those whose existence follows from the statement of the problem and common sense knowledge. When we circumscribe the first order logic statement of the problem together with the common sense facts about boats etc., we will be able to conclude that there is no bridge or helicopter. ``Aha'', you say, ``but there won't be any oars either''. No, we get out of that as follows: It is a part of common knowledge that a boat can be used to cross a river unless there is something wrong with it or something else prevents using it, and if our facts don't require that there be something that prevents crossing the river, circumscription will generate the conjecture that there isn't. The price is introducing as entities in our language the ``somethings'' that may prevent the use of the boat.

If the statement of the problem were extended to mention a bridge, then the circumscription of the problem statement would no longer permit showing the non-existence of a bridge, i.e. a conclusion that can be drawn from a smaller collection of facts can no longer be drawn from a larger. This nonmonotonic character of circumscription is just what we want for this kind of problem. The statement, ``There is a bridge a mile upstream, and the boat has a leak.'' doesn't contradict the text of the problem, but its addition invalidates the Amarel representation.

In the usual sort of puzzle, there is a convention that there are no additional objects beyond those mentioned in the puzzle or whose existence is deducible from the puzzle and common sense knowledge. The convention can be explicated as applying circumscription to the puzzle statement and a certain part of common sense knowledge. However, if one really were sitting by a river bank and these six people came by and posed their problem, one wouldn't take the circumscription for granted, but one would consider the result of circumscription as a hypothesis. In puzzles, circumscription seems to be a rule of inference, while in life it is a rule of conjecture.

Some have suggested that the difficulties might be avoided by introducing probabilities. They suggest that the existence of a bridge is improbable. The whole situation involving cannibals with the postulated properties cannot be regarded as having a probability, so it is hard to take seriously the conditional probability of a bridge given the hypotheses. More to the point, we mentally propose to ourselves the normal non-bridge non-sea-monster interpretation before considering these extraneous possibilities, let alone their probabilities, i.e. we usually don't even introduce the sample space in which these possibilities are assigned whatever probabilities one might consider them to have. Therefore, regardless of our knowledge of probabilities, we need a way of formulating the normal situation from the statement of the facts, and nonmonotonic reasoning seems to be required. The same considerations seem to apply to fuzzy logic.

Using circumscription requires that common sense knowledge be expressed in a form that says a boat can be used to cross rivers unless there is something that prevents its use. In particular, it looks like we must introduce into our ontology (the things that exist) a category that includes something wrong with a boat or a category that includes something that may prevent its use. Incidentally, once we have decided to admit something wrong with the boat, we are inclined to admit a lack of oars as such a something and to ask questions like, ``Is a lack of oars all that is wrong with the boat?''.

Some philosophers and scientists may be reluctant to introduce such things, but since ordinary language allows ``something wrong with the boat'' we shouldn't be hasty in excluding it. Making a suitable formalism is likely to be technically difficult as well as philosophically problematical, but we must try.

We challenge anyone who thinks he can avoid such entities to express in his favorite formalism, ``Besides leakiness, there is something else wrong with the boat''. A good solution would avoid counterfactuals as this one does.

Circumscription may help understand natural language, because if the use of natural language involves something like circumscription, it is understandable that the expression of general common sense facts in natural language will be difficult without some form of nonmonotonic reasoning.

next up previous

John McCarthy
Tue May 14 00:04:52 PDT 1996