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\address 
Professor Zenon W. Pylyshyn
Director, Artificial Intelligence
\hskip.5cm and Robotics
The Canadian Institute for Advanced Research
The University of Western Ontario
London, Ontario N6A 5C2

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Dear Professor Pylyshyn:

	David Etherington has written about circumscription, my proposal
for non-monotonic inference, both in collaboration with Ray Reiter and
on his own.  He has also given a lecture here at Stanford.

	His main interest has been in clarifying the concepts of non-monotonic
reasoning and determining the limitations of their logical mechanisms.  In
this he has addressed the right issues.

	He takes the mathematical aspects of the theory seriously and
works on his ideas until they become precise results.

	His lecture here at Stanford was very clear and well motivated.

	Besides referring to my own memory of his work, I have consulted
my colleague Vladimir Lifschitz, who has looked carefully at Etherington's
mathematical work.

	I think he is an excellent choice as an Institute Scholar.


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Sincerely,    

John McCarthy    
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