According to a “presence/absence hypothesis,” the hippocampus is not necessary for the formation of learned associations between currently present stimuli and responses (as in classical conditioning), but is necessary whenever a stimulus, if it is to activate a particular response, must first activate a memory-representation of something not present in the here-and-now. The distinction between responses made to present stimuli as opposed to (memories of) absent stimuli was first stressed by Romanes (1889), but we find evidence in the target article that supports the relevance of this distinction to our understanding of hippocampal functioning.