It would be desirable to have a definition of ``commonsense knowledge'' that characterized this concept precisely. This is not yet possible, and may never be, but the following definition is an attempt to distinguish commonsense knowledge from basic cognitive skills such as manipulation, vision, and language on the one hand, and expert knowledge of electronics, mathematics, or history on the other. The abundant qualifiers in the definition are necessary because of the fuzziness of the phenomenon itself:
\begin{quote}\em Commonsense knowledge is knowledge about the structure of the external world that is acquired and applied without concentrated effort by any normal human that allows him or her to meet the everyday demands of the physical, spatial, temporal, and social environment with a reasonable degree of success. \end{quote}Commonsense knowledge is useful exactly because it is a description of the environment that is maintained at very low cost. Planning and action can then take place within a rich description of the context that need not be constructed for the particular occasion.