[5] Kaplan began as an undergraduate at UCLA in 1951, admitted on academic probation "owing to poor grades.
He was the last doctoral student supervised by Rudolf Carnap, receiving his PhD in 1964 with a thesis entitled Foundations of Intensional Logic.
His work continues the strongly formal approach to philosophy long associated with UCLA (as represented by mathematician-logician-philosophers such as Alonzo Church and Richard Montague).
In most years, Kaplan teaches an upper division course on philosophy of language, focusing on the work of either Gottlob Frege, Bertrand Russell, or P. F. Strawson.
[citation needed] Kaplan's insights center on two key distinctions, which may be seen as responses to the inability of Frege's semantics to deal with context-sensitivity in language.
First, in place of Frege's categories of Sinn and Bedeutung (typically translated as "sense" and "reference"), Kaplan introduces the notions of character and content.
Character defines a function associated by convention with an expression, which takes contextual elements as arguments and yields content as values.
On the other hand, an expression is directly referential if, and only if its content defines a constant function from circumstances of evaluation to extension.
Roughly, the puzzle here arises as indexicals are thought to be directly referential, i.e., they do not refer by means of a Fregean Sinn.
While this approach to proper names is not novel (John Stuart Mill being an early advocate), Frege's Puzzle is thought to cast doubt on any such account.
Many philosophers have attempted to deal with this issue (notably Joseph Almog, David Braun, Michael Devitt, John Perry, Nathan Salmon, Scott Soames, and Howard Wettstein), but no solution has been widely accepted.
For example, (using a propositional attitude clause), if one quantifies into the statement "Ralph believes that Ortcutt is a spy," the result is (partly formalized): In short, Kaplan attempts (among other things) to provide an apparatus (in a Fregean vein) that allows one to quantify into such intensional contexts even if they exhibit the kind of substitution failure that Quine discusses.
The program was initially developed to complement the logic text of Donald Kalish and Richard Montague, and the derivations module therefore uses their distinctive natural deduction system.