This rule of thumb will be shown to be wrong as it stands; but 2 We include superlatives ('tallest'), comparatives ('taller'), and conditionals ('if') in this category because they have nondefault (i.e., non-upward entailing) properties -for instance, 'he is the tallest father' does not entail 'he is the tallest man'.
they also require special treatment when considering entailment relations.