Unicorns!

Apropos the whole business of abstractness and nouns' denotations, there is a syndrome that's either a high-level abstraction or some type of strange nominal mutation. 'Horse can mean this one horse right here, or it can mean the abstract concept, as in 'Horse = hoofed mammal of family Equidae'. Same with the word 'horn'; same with 'forehead'. All these can be abstracted from particulars, but we still know they came from particulars. Except what about a unicorn, which seems to result from the combination of the concepts 'horse', 'horn', and 'forehead' and thus has its whole origin in the concatenation of abstractions? Meaning we can conjoin and manipulate abstractions to form entities whose nouns have no particular denotations at all. Here the big problem becomes: In what way can we say a unicorn exists that is fundamentally different, less real, than the way abstractions like humanity or horn or integer exist?

—David Foster Wallace, from Everything & More