The privative a (or a privativum) is the prefixa- expressing negation (e.g. a-theism, a-neurism). Originally described for the grammar of Ancient Greek, it goes back to a PIE syllabic nasal *n̥-, the zero ablaut grade of the negation *ne. For this reason, it appears as an- before vowel (e.g. an-alphabetism). The same prefix appears in Sanskrit, also as a-, an-. In Latin, the cognate prefix is in-, and in West Germanic languages (including English) it is un-. In North Germanic languages, the -n- has disappeared and Swedish has o-, whereas Danish and Norwegian have u-.
Opposites in the sense of 'privatives' and 'positives' are' blindness' and 'sight'; in the sense of affirmatives and negatives, the propositions 'he sits', 'he does not sit'.
Blindness is a 'privative', to be blind is to be in a state of privation, but is not a 'privative'.
In the case of 'positives' and 'privatives', if the subject does not exist at all, neither proposition is true, but even if the subject exists, it is not always the fact that one is true and the other false.