1. Alston, William: 1964, Philosophy of Language. Prentice-Hall, Chapter 5.
2. Bolinger, Dwight: 1972, Degree Words, Mouton.
3. Goguen, J. A.: 1971, ‘The Logic of Inexact Concepts’, Synthese.
4. unpublished paper, Pyschology Dept., U. of California, Berkeley;ER Heider,1971
5. unpublished paper, Philosophy Dept., U. of Toronto;H Herzberger,1971