Such an idea has led to a number of theories which highlight the duality of process and
concept. Davis (1975) noted children may not distinguish between the name of a symbol
and the underlying process. Skemp (1979) proposed a general “varifocal theory” in
which a schema seen as a whole is a concept and a concept seen in detail is a schema.
Greeno (1983) focused on the notion of “conceptual entities” which may be used as
inputs to other procedures. More recently, Dubinsky (1991) speaks of encapsulation of
process as object, Sfard (1991) of reification of process as object, and Gray & Tall
(1994) see the symbol as pivot between process and concept—the notion of procept.