Are Some Stimuli “More Equivalent” Than Others?

The model of stimulus equivalence proposed by Sidman and colleagues (e.g., Sidman, 1986, 1994; Sidman & Tailby, 1982) offered operational criteria to iden-tify symbolic functions in experimentally observable behaviors and to simulate in laboratory symbolic relations that occur in natural situations. The model offered, for instance, behavioral tests based on matching to sample procedures to verify if a relation arbitrarily established between dissimilar stimuli has the logical properties of reflex-ivity, symmetry and transitivity. If so, these stimuli will constitute a class of equivalent stimuli in which each member will be sub-stitutable by the others, simulating stimuli that are symbolic related, for example, in natural languages