Bertrand Russell,
On Denoting
(1905)
“ The distinction between acquaintance and knowledge about is the distinction between the things we have presentations of, and the things we only reach by means of denoting phrases. It often happens that we know that a certain phrase denotes unambiguously, although we have no acquaintance with what it denotes; this occurs in the above case of the center of mass. In perception we have acquaintance with objects of perception, and in thought we have acquaintance with objects of a more abstract logical character ”
