The article by Murray Sidman and William Tailby explores the distinction between conditional discrimination and matching to sample in the context of stimulus equivalence. Conditional discrimination involves learning to select one stimulus based on the presence of another, while matching to sample involves selecting a stimulus that matches a sample. The authors use a paradigm with three sets of stimuli (A, B, and C) to test whether conditional discrimination generates equivalence relations. If subjects can match Set-B and Set-C stimuli based on Set-A samples, it suggests that A1 and B1 are equivalent, and A2 and B2 are equivalent. The study expands this paradigm by introducing a fourth set (D) and teaching subjects to select Set-C comparisons based on Set-D samples. This generates new conditional and matching relations, confirming the formation of four-member stimulus classes. The experiment uses Greek letters and their names to avoid pretests and ensures that the stimuli are unfamiliar to the subjects. The results show that all subjects successfully learned the conditional discriminations and matched the stimuli, confirming the emergence of equivalence relations. The study also tests for symmetry and transitivity, providing evidence that the conditional discriminations generated not only conditional but also equivalence relations.The article by Murray Sidman and William Tailby explores the distinction between conditional discrimination and matching to sample in the context of stimulus equivalence. Conditional discrimination involves learning to select one stimulus based on the presence of another, while matching to sample involves selecting a stimulus that matches a sample. The authors use a paradigm with three sets of stimuli (A, B, and C) to test whether conditional discrimination generates equivalence relations. If subjects can match Set-B and Set-C stimuli based on Set-A samples, it suggests that A1 and B1 are equivalent, and A2 and B2 are equivalent. The study expands this paradigm by introducing a fourth set (D) and teaching subjects to select Set-C comparisons based on Set-D samples. This generates new conditional and matching relations, confirming the formation of four-member stimulus classes. The experiment uses Greek letters and their names to avoid pretests and ensures that the stimuli are unfamiliar to the subjects. The results show that all subjects successfully learned the conditional discriminations and matched the stimuli, confirming the emergence of equivalence relations. The study also tests for symmetry and transitivity, providing evidence that the conditional discriminations generated not only conditional but also equivalence relations.