WordNet sense 1 (the character of the positive electric pole):

positiveness

WordNet sense 2 (a quality or state characterized by certainty or acceptance or affirmation and dogmatic assertiveness):

positivism

WordNet sense 3 (the quality of being encouraging or promising of a successful outcome):

advantageousness, favorableness, favourableness, profitableness

WordNet sense 4 (the quality of being undeniable and not worth arguing about):

incontrovertibility, incontrovertibleness