A dictionary that only defines words using other words never actually means anything

A computer can manipulate the symbol 'apple' flawlessly, matching it to 'fruit' and 'red' and 'tree', yet never once connect that symbol to an actual apple's taste, weight or smell. Stevan Harnad named this gap the symbol grounding problem: a purely symbolic system seems to be forever defining words in terms of other words, with the chain of meaning never touching anything real, which is precisely what embodied senses give human understanding for free.

— Stevan Harnad, The Symbol Grounding Problem — Physica D, 1990