Forward chaining (or forward reasoning) is one of the two main methods of reasoning when using an inference engine and can be described logically as repeated application of modus ponens. Forward chaining is a popular implementation strategy for expert systems, business and production rule systems. The opposite of forward chaining is backward chaining.
Forward chaining starts with the available data and uses inference rules to extract more data (from an end user, for example) until a goal is reached. An inference engine using forward chaining searches the inference rules until it finds one where the antecedent (If clause) is known to be true. When such a rule is found, the engine can conclude, or infer, the consequent (Then clause), resulting in the addition of new information to its data.
Inference engines will iterate through this process until a goal is reached.
Suppose that the goal is to conclude the color of a pet named Fritz, given that he croaks and eats flies, and that the rule base contains the following four rules:
If X croaks and X eats flies - Then X is a frog
If X chirps and X sings - Then X is a canary
If X is a frog - Then X is green
If X is a canary - Then X is blue
Let us illustrate forward chaining by following the pattern of a computer as it evaluates the rules.
Assume the following facts:
Fritz croaks
Fritz eats flies
With forward reasoning, the inference engine can derive that Fritz is green in a series of steps:
Since the base facts indicate that "Fritz croaks" and "Fritz eats flies", the antecedent of rule #1 is satisfied by substituting Fritz for X, and the inference engine concludes:
Fritz is a frog
The antecedent of rule #3 is then satisfied by substituting Fritz for X, and the inference engine concludes:
Fritz is green
The name "forward chaining" comes from the fact that the inference engine starts with the data and reasons its way to the answer,
as opposed to backward chaining, which works the other way around.
In the derivation, the rules are used in the opposite order as compared to backward chaining.
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The Rete algorithm (ˈriːtiː , ˈreɪtiː , rarely ˈriːt , rɛˈteɪ ) is a pattern matching algorithm for implementing rule-based systems. The algorithm was developed to efficiently apply many rules or patterns to many objects, or facts, in a knowledge base. It is used to determine which of the system's rules should fire based on its data store, its facts. The Rete algorithm was designed by Charles L. Forgy of Carnegie Mellon University, first published in a working paper in 1974, and later elaborated in his 1979 Ph.
In the field of artificial intelligence, an inference engine is a component of the system that applies logical rules to the knowledge base to deduce new information. The first inference engines were components of expert systems. The typical expert system consisted of a knowledge base and an inference engine. The knowledge base stored facts about the world. The inference engine applies logical rules to the knowledge base and deduced new knowledge. This process would iterate as each new fact in the knowledge base could trigger additional rules in the inference engine.
Backward chaining (or backward reasoning) is an inference method described colloquially as working backward from the goal. It is used in automated theorem provers, inference engines, proof assistants, and other artificial intelligence applications. In game theory, researchers apply it to (simpler) subgames to find a solution to the game, in a process called backward induction. In chess, it is called retrograde analysis, and it is used to generate table bases for chess endgames for computer chess.
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