  A fundamental reason for the failure of the introduction form property in classical logic is the indeterminism inherent in the laws for disjunction. A∨B can be inferred from  A as well as from B. Therefore, if the disjunction laws were the only way of inferring A∨B, the derivability of A∨¬A, which is a key principle of classical logic, would entail that of either A or of ¬A, which is absurd. A way out of this difficulty is to abolish indeterministic disjunction and use instead its classical de Morgan equivalent ¬(¬A ∧¬B). This leads essentially to a logic without proper disjunction. In the quantifier case, there would be no proper existential quantifier either, as ∃xA would be understood in the sense of ¬∀x¬A. If one is prepared to accept this restriction, then certain harmony principles can be formulated for classical logic.