I vaguely recall reading, years ago, a passage from, I believe, Peter Geach, in the context of research I was doing on the "Is-Ought" problem Geach was considering an argument that purported to move from a premise with the copula "is" to a conclusion with the copula "ought". The argument in question was something like this:

Jones is a sea captain.
Under conditions C a sea captain has a duty to do O.
Conditions C obtain.
Jones has a duty to do O
Jones ought to do O.

Can anyone help me to find the source of Geach's discussion?

Ken Kipnis
kkipnis@hawaii.edu