Arkady and Boris Strugatsky. Roadside picnic (№ 786)

Usually a trivial definition is used: reason is that part of man which distinguishes its activity from animal activity. Sort of, you know, an attempt to distinguish the owner from the dog who understands everything, just can't speak. However, this flat definition suggests a more witty. Based on bitter observation of the above-mentioned human activities. For example: reason is the ability of a living creature to perform unreasonable or unnatural acts.