The ability to represent same-different relations is a condition for abstract thought. However, there is mixed evidence for when this ability develops, both ontogenetically and phylogenetically. Apparent success in relational reasoning may be evidence for conceptual understanding or may be due to low-level, perceptual strategies. We introduce a method to discriminate these possibilities by pitting two conditions that are perceptually matched but conceptually different: in a “fused” condition, same and different objects are joined, creating single objects that have the same perceptual features as the pairs in the “relational” condition. However, the “fused” objects do not provide evidence for the relation. Using this method in a causal task provides evidence for genuine conceptual understanding. This novel technique offers a simple manipulation that may be applied to a variety of existing match-to-sample procedures used to assess same-different reasoning to include in future research with non-human animals, as well as human infants.