Supplement to Defeasible Reasoning

AGM Postulates

Where K is a belief state, K*A represents the set of beliefs resulting from revising K with new belief A.

  • (K*1) K*A is closed under logical consequence.
  • (K*2) A belongs to K*A.
  • (K*3) K*A is a subset of the logical closure of K ∪ {A}.
  • (K*4) If ¬A does not belong to K, then the closure of K ∪ {A} is a subset of K*A.
  • (K*5) If K*A is logically inconsistent, then either K is inconsistent, or A is.
  • (K*6) If A and B are logically equivalent, then K*A = K*B.
  • (K*7) K*(A & B) is a subset of the logical closure of K*A ∪ {B}.
  • (K*8) If ¬B does not belong to K*A, then the logical closure of K*AB is a subset of K*(A & B).

Copyright © 2013 by
Robert Koons <>

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