(20th-August-2020)
• A primitive atom is an atom that is stated as an atomic clause when it is true. A derived atom is one that uses rules to define when it is true. Typically the designer writes axioms for the derived atoms and then expects a user to specify which primitive atoms are true. Thus, the derived atoms will be inferred as necessary from the primitive atoms and other atoms that can be derived.
• The designer of an agent must make many decisions when designing a knowledge base for a domain. For example, consider two propositions,a andb, both of which are true. There are many choices of how to write this. A designer could specify both a and b as atomic clauses, treating both as primitive. A designer could have a as primitive and b as derived, stating a as an atomic clause and giving the rule b ←a. Alternatively, the designer could specify the atomic clause b and the rule a ←b, treating b as primitive and a as derived. These representations are logically equivalent; they cannot be distinguished logically. However, they have different effects when the knowledge base is changed. Suppose a was no longer true for some reason. In the first and third representations, b would still be true, and in the second representation b would no longer true.
• A causal model, or a model of causality, is a representation of a domain that predicts the results of interventions. An intervention is an action that forces a variable to have a particular value; that is, it changes the value in some way other than manipulating other variables in the model.
• To predict the effect of interventions, a causal model represents how the cause implies its effect. When the cause is changed, its effect should be changed. An evidential model represents a domain in the other direction - from effect to cause. Note that we do not assume that there is "the cause" of an effect; rather there are many propositions, which together make the effect true.
• For example, the electrical domain depicted in Figure 1.8. In this domain, switch s3 is up and light l2 is lit. There are many different ways to axiomatize this domain. Example 5.5 contains causal rules such as
• lit_l2 ←up_s3 ∧live_w3.
• Alternatively, we could specify in the evidential direction:
• up_s3 ←lit_l2.
• live_w3 ←lit_l2.
• These are all statements that are true of the domain.
• Suppose that wire w3 was live and someone put switch s3 up; we would expect that l2 would become lit. However, if someone was to make s3 lit by some mechanism outside of the model (and not by flipping the switch), we would not expect the switch to go up as a side effect.
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