@incollection{eprints2159,
       booktitle = {Enciclopedia Filosofica},
           title = {Validit{\`a}},
            year = {2006},
          volume = {12},
       publisher = {Bompiani},
          author = {Stefano Gattei and Enrico De Mas},
           pages = {11981--11982},
             url = {http://eprints.imtlucca.it/2159/}
}