The 'be' verb (more properly, copula) has two forms, positive and negative, artificially separating everything into binary divisions and prescribing some unchanging 'essence' to every 'thing' (noun, for example). People argue and even go to war over what some 'thing' (or situation) 'is' or 'isn't'--essentially what we label that as. Various semanticists have created other systems that include the imperative and ubiquitous gray zone, the world of maybes and overlapping linguistic 'truths'. A favorite writer of mine (Bob Wilson) preferred the label 'maybe logic'.
Basically, whenever something 'is' multiple, seemingly contradictory but nonetheless true 'things' (labels), then the binary, Aristotelean division simply doesn't work. Always look for a better model (as physicists and other scientists do) to include more of the data and remove more of the bias. At very least, be aware of the semantic trappings and pitfalls of your own language.