Describe a banana. It's yellow, perhaps with some green edges. When peeled, it has a smooth, soft, mushy texture. It tastes sweet, maybe a little creamy.
And it smells like... well, it smells like a banana.
Every sense has its own “lexical field,” a vast palette of dedicated descriptive words for colors, sounds, tastes, and textures. But smell? In English, there are only three dedicated smell words—stinky, fragrant, and musty—and the first two are more about the smeller's subjective experience than about the smelly thing itself.
All of our other scent descriptors are really descriptions of sources: We say that things smell like cinnamon, or roses, or teen spirit, or napalm in the morning. The other senses don't need these linguistic workarounds. We don't need to say that a banana “looks like lemon;” we can just say that it's yellow. Experts who work in perfume or wine-tasting industries may use more metaphorical terms like decadent or unctuous, but good luck explaining them to a non-expert who's not familiar with the jargon.
Some scientists have taken this as evidence that humans have relegated smell to the sensory sidelines, while vision has taken center-field. It's a B-list sense, deemed by Darwin to be “of extremely slight service.” Others have suggested that smells are inherently indescribable, and that “olfactory abstraction is impossible.” Kant wrote that “Smell does not allow itself to be described, but only compared through similarity with another sense.” Indeed, when Jean-Baptiste Grenouille, the protagonist of Perfume: The Story of a Murderer can unerringly identify smells, remember them, and mix and match them in his head, he seems disconcerting and supernatural to us, precisely because we suck so badly at those tasks.