In his works on zoological classification, Carl Linnaeus was keenly interested in the relationship between apes and humans and he repeatedly reworked his definitions and taxonomy. His interest in this subject was particularly stimulated by the fact that it was a problematic area. To aid his research, Linneaus also kept live monkeys in his botanical garden. Studying these animals close at hand was very productive. It helped him to refine his definitions and it clarified his efforts to understand the boundary between humans and other animals.