First and second class objects. In , a real number may appear in an expression or be assigned to a variable, and either may appear as an actual parameter in a procedure call. A procedure, on the other hand, may only appear in another procedure call either as the operator (the most common case) or as one of the actual parameters. There are no other expressions involving procedures or whose results are procedures. Thus in a sense procedures in are second class citizens—they always have to appear in person and can never be represented by a variable or expression (except in the case of a formal parameter)...
〔Christopher Strachey, "Fundamental Concepts in Programming Languages" in ''Higher-Order and Symbolic Computation'' 13:11 (2000); though published in 2000, these are notes from lectures Strachey delivered in August, 1967〕