On a distinction between hypothetical constructs and intervening variables.

As the thinking of behavior theorists has become more sophisticated and selfconscious, there has been considerable discussion of the value and logical status of so-called ‘intervening variables. ’ Hull speaks of “symbolic constructs, intervening variables, or hypothetical entities” (5, p. 22) and deals with them in his theoretical discussion as being roughly equivalent notions. At least, his exposition does not distinguish among them explicitly. In his presidential address on behavior at a choice point, Tolman inserts one of Hull’s serial conditioning diagrams (11, p. 13) between the independent variables (maintenance schedule, goal object, etc.) and the dependent variable (‘behavior ratio’) to illustrate his concept of the intervening variable. This would seem to imply that Tolman views his ‘intervening variables’ as of the same character as Hull’s. In view of this, it is somewhat surprising to discover that Skinner apparently feels that his formulations have a close affinity to those of Tolman, but are basically dissimilar to those of Hull (10, p. 436, 437). In advocating a theoretical structure which is ‘descriptive’ and ‘positivistic,’ he suggests that the model chosen by Hull (Newtonian mechanics) is not the most suitable model for purposes of behavior theory; and in general is critical of the whole postulate-deductive approach. Simultaneously with these trends, one can still observe among ‘tough-minded’ psychologists the use of words such as ‘unobservable’ and ‘hypothetical’ in an essentially derogatory manner, and an almost compulsive fear of passing beyond the direct colligation of observable data. ‘Fictions’ and ‘hypothetical entities’ are sometimes introduced into a discussion of theory with a degree of trepidation and apology quite unlike the freedom with which physicists talk about atoms, mesons, fields, and the like. There also seems to be a tendency to treat all hypothetical constructs as on the same footing merely because they are hypothetical; so that we find people arguing that if neutrons are admissible in physics, it must be admissible for us to talk about, e.g., the damming up of libido and its reversion to earlier channels. The view which theoretical psychologists take toward intervening variables and hypothetical constructs will of course profoundly influence the direction of theoretical thought. Furthermore, what kinds of hypothetical constructs we become accustomed to thinking about will have a considerable impact upon theory creation. The present paper aims to present what seems to us a major problem in the conceptualization of intervening variables, without claiming to offer a wholly satisfactory solution. Chiefly, it is our aim here to make a distinction between two subclasses of intervening variables, or we prefer to say, between ‘intervening variables’ and ‘hypothetical constructs’ which we feel is fundamental but is currently being neglected. We shall begin with a common-sense distinction, and proceed later to formulations of this distinction which we hope will be more rigorous. Naively, it would seem that there is a difference in logical status between constructs which involve the hypothesization of an entity, proc-