 Mally’s informal postulates (i)–(iii) and (v) are conditionals or negations of conditionals, i.e., of the form “if … then —” or “not: if … then —.” Føllesdal and Hilpinen (1981, pp. 5-6) have suggested that such conditionals should not be formalized in terms of material implication and that some sort of strict implication would be more appropriate. But this suggestion is not altogether satisfactory, for both A → !A and A ↔ !A are derivable in the very weak system S0.90 plus I′ and III′, where → is the symbol of strict  implication.[7]