Experiments with 9 rhesus monkeys (Macaca mulatta) showed, for the first time, that abstract-concept learning varied with the training stimulus set size. In a same/different task, monkeys required to touch a top picture before choosing a bottom picture (same) or white rectangle (different) learned rapidly. Monkeys not required to touch the top picture or presented with the top picture for a fixed time learned slowly or not at all. No abstract-concept learning occurred after 8-item training but progressively improved with larger set sizes and was complete following 128-item training. A control monkey with a constant 8-item set ruled out repeated training and testing. Contrary to the unique-species account, it is argued that different species have quantitative, not qualitative, differences in abstract-concept learning.Abstract concepts are the basis of much of our so-called higher order cognitive processing (e.g., language and mathematics). Abstract concepts are rules about relationships such as identity or sameness. Children apparently develop cognition in stages and expand their abstract concept of sameness to include number length, area, and volume (Piaget & Inhelder, 1966/1969. Indeed, William James (1890/1950) declared that our "sense of sameness is the very keel and backbone of our thinking" (p. 459). In the laboratory, the abstract concept of sameness is usually studied in matching-to-sample (MTS) or same/different (S/D) tasks. In MTS, subjects typically view a sample stimulus and then choose one of two comparison stimuli, with the correct choice being the comparison that matches the sample. In S/D, subjects view a pair of stimuli and then make one of two responses to indicate whether the stimuli are the same or different. The determination of abstractconcept learning in both cases is accurate performance with novel test stimuli. Accurate performance with novel stimuli means that the subject has learned an abstract rule that transcends the particular training stimuli. Such transfer performance is what makes abstract-concept learning unique and different from other forms of concept learning.Abstract concepts are different from what are called "natural" concepts. Natural concepts are categories of items bound by absolute features of objects such as cars, chairs, flowers, person, water, or trees (e.g., Bhatt, Wasserman, Reynolds, & Knauss, 1988;Herrnstein, Loveland, & Cable, 1976;Roberts & Mazmanian, 1988). Natural concepts involve learning specific features shared by a class or category of items (e.g., prototype). By contrast, abstract concepts do not involve learning specific stimulus features. Instead, they involve learning the relationships between items. Thus, abstract concepts involve relational learning as opposed to the item-specific learning of natural-concept learning.