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C00002 00002	%3Concepts as objects%1
C00004 00003	1. concepts meaningful in approximate theories
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%3Concepts as objects%1

	The discussion in this paper is based on an example.
(McCarthy 1977a) contains a more systematic discussion.

	Opening a safe with a key or with a combination can
be expressed by formulas ({eq e1}) and ({eq e2}) respectively.

!!e1:	%2(∀ person safe s)(open(safe,result(opens1(person,key(safe),safe),s)))%1

!!e2:	%2(∀ person safe s)(open(safe,result(opens2(person,combination(safe),safe),s)))%1

	The meaning of the formulas is that the safe will be open in
the situation that results from the situation ⊗s and the occurrence
of the event of %2person%1 opening %2safe%1 with either ⊗key(safe) or
⊗combination(safe).  We use the names ⊗opens1 and ⊗opens2 for the two
kinds of actions of opening, but the treatments are entirely
parallel.  Knowledge of the combination is not involved in ({eq e2}), because
the safe would still open if the right combination were dialed at
random.

1. concepts meaningful in approximate theories