The big mistake, the great illusion, of the materialistically minded people, is that they believe a definition or a description can fully explain the essence of a thing.
I have often illustrated the grotesque nature of this belief in the past by pointing out the fact that a school of Greek philosophers once sought to define a human being and finally found that a human being is a creature with two legs and no feathers. Now, that is undoubtedly true. One can say that it is an absolutely correct definition. The next day, someone who understood the definition brought with him a plucked rooster and said: “This is a creature with two legs and no feathers, so it must be a human being.”
Source (German): Rudolf Steiner – GA 253 – Probleme des Zusammenlebens in der Anthroposophischen Gesellschaft – Dornach, September 11, 1915 (page 33)