Jul 29, 2010

DiagnostiCar (4) - Abductive Reasoning


finally I had some time to write about abductive reasoning, the core of the DiagnostiCar expert system. As you may know from my previous posts (means you're tough and reached this point :P), the abductive reasoning tries to infer what facts implies a given situation.

To build such a reasoning we must start with the conclusion and try to prove it. Let's say that we have the following scenario:

known mortal(X) <- human(X).

known human(socrates).

How abductive reasoning behaves when tries to find out what things are mortal?

The first step is to check if mortal(X) is a tautology such as:

known mortal(xerxes).

As in our example there isn't such tautology the reasoning agent must try to find a rule which implies that something is mortal. In our case proceeds that mortal(X) <- human(X) so to prove that something is mortal we have to prove that it is human. The first step is then repeated and we reach the tautology human(socrates) which asserts that Socrates is mortal.

Note that we have no difference with we try to use Prolog directly:

mortal(X) :-


Now consider a non trivial example. Imagine that we want to prove that mortal(aristoteles) but we may let the reasoner conclude that such affirmation is only possible if Aristoteles is a human. Pretty smart huh? That's what abductive reasoning is capable of. To build such a predicate we will need to know not only what are proving but also the user id (to ask questions when possible) and a list to yield what is being assumed.

Now lets try to prove a goal using the following rules (together with the Prolog code):

1 - Check if the goal is a tautology:

abduction(_, Goal, _) :-

2 - Check equalities:

abduction(_, Value == Value, _) :-
    Value = Value.

abduction(_, Value \= Value, _) :-
    Value \= Value.

3 - Check if the goal is implied by something else:

abduction(Uid, Goal, Assuming) :-
    known(Goal <- Cause),
    abduction(Uid, Cause, Assuming).

4 - If the goal is an or or an and conjunction:

abduction(Uid, Left & Right, Assuming) :-
    abduction(Uid, Left, Assuming),
    abduction(Uid, Right, Assuming).

abduction(Uid, Left v _, Assuming) :-
    abduction(Uid, Left, Assuming).

abduction(Uid,_ v Right,Assuming) :-
    abduction(Uid, Right, Assuming).

5 - Ask the user:

abduction(Uid, Goal, _) :-
    ask(Uid,Question,Answer). % Defined in a previous post

6 - Make an assumption:

abduction(_, Goal, Assuming) :-
    member(Goal, Assuming).
    % Says that the goal must be assumed to reach the goal.

This does it's task but we still have some problems. The first one is the lack of constraints. This reasoning doesn't consider if you assume something that implies in a contradiction. e.g.: known false <- mortal(X) & immortal(X). Another problem we can observe is that the system doesn't follow the Occam's razor principle, which affirms that between two valid explanation the simplest tends to be true. Finally we have a crucial problem is a infinite loop occurence when you try to prove the negation of something and the goal is a tautology.

Let's solve those problems by parts. The first can be solved with a predicate which proves the goal and doesn't reach a contradiction at the same time:

reason(Uid, Goal, Assuming) :-
    abductive_reasoning(Uid, Goal, Assuming),
    not(abductive_reasoning(Uid, false, Assuming)).

But that predicate falls directly to the third problem. To solve this problem, the must intuitive solution is do a search starting with a empty list of assumptions and try incrementing the list size until it reachs a cutoff that you may assume that there is no explanation bigger than that. A simple and clear implementation of that would be:

abductive_reasoning(Uid, Goal, Assuming) :-
    abduction(Uid, Goal, Assuming, Max).

abduction(Uid, Goal, Assuming, Depth) :-
    Depth >= 0,
    Len is Max - Depth,
    length(Assuming, Len),
    abduction(Uid, Goal, Assuming).

abduction(Uid, Goal, Assuming, Depth) :-
    NextStep is Depth - 1,
    NextStep >= 0,
    abduction(Uid, Goal, Assuming, NextStep).

It is not the most efficient code to handle code but works fine. You can avoid process a search path again by creating a dynamic predicate and asserting when a branch of the search is truth or false. It may use a lot of memory but it helps to execute faster.

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