Books on Amazon
Gradual/Degree/Yes-No/Explanation/Method/Definition/Hintikka: Thesis: seemingly dichotomous concepts can often be explained better if one considers them as gradual.
Definability/Rantala/Hintikka: Rantala thesis: we do not start with asking when a theory clearly defines a concept, but how much freedom the theory leaves to the concept.
Determinateness/Hintikka: is a gradual issue, and definability sets in when the uncertainty disappears. This is an elegant correspondence of model theory.
Qualitative/comparative/Hintikka: by the assumption that a property is gradual, one can transform a qualitative concept into a comparative one. Then we do not only have to deal with yes-no questions.
Jaakko and Merrill B. Hintikka
The Logic of Epistemology and the Epistemology of Logic Dordrecht 1989
J. Hintikka/M. B. Hintikka
Untersuchungen zu Wittgenstein Frankfurt 1996