Olfaction is often referred to as a multidimensional sense. It is multidimensional in that 1000 different receptor types, each tuned to particular odor aspects, together contribute to the olfactory percept. In humans, however, this percept is nearly unidimensional. Humans can detect and discriminate countless odorants, but can identify few by name. The one thing humans can and do invariably say about an odor is whether it is pleasant or not. We argue that this hedonic determination is the key function of olfaction. Thus, the boundaries of an odor object are determined by its pleasantness, which—unlike something material and more like an emotion—remains poorly delineated with words.