100% fake leather

One of the knitting websites I use has the parts to make handbags, including firm bases which you can crochet around to get a more sturdy design. I refuse to believe they didn't know what they were doing when they described this as 'a lovely and practical round bottom', but that's not why I'm showing it to you. 

The material is described as 100% PU leather. PU means polyurethane, which means that it's artificial leather, not made from animal skin. This also means that it's absolutely inaccurate to describe it as being 'made of 100% leather', as they do immediately above. 

Or is it? 

Adjectives can be classified as being intersective, subsective, and privative (there's a couple more too but that's all we need for now). 

Intersective ones are so-called because their meaning is the intersection of two things: the adjective and the noun they describe. So, semanticist Barbara Partee gives carnivorous as an example because a carnivorous mammal describes things that are in the intersection of the set of 'things that are carnivorous' and 'things that are mammals'. 

Subsective adjectives are a bit trickier but not much: they describe a subset of the noun. Partee's example is skillful, because a skillful violinist is a subset of violinists - the ones who are skillful. It doesn't describe someone who is both skillful (independently) and a violinist, because it doesn't describe their skillfulness in any other field (e.g. skiing), only violining. (This is also the basis of my good dalek observation, by the way.)

Then we have privative adjectives, which are neither an intersection nor a subset, but actually negate the property of the noun they describe. A fake diamond is not a diamond at all, only something pretending to be one. In our round bottom example, it's not made of leather at all - it's fake leather. 

But Partee cites the acceptability of both the following examples to show that this cannot be the whole story: 

A fake gun is not a gun. 

Is that gun real or fake? 

In the second example, the item in question is still described as a gun. And actually most of the examples that ran through my mind for a privative adjective are more like this: a fictitious detective is still a detective (unless you want to tell Poirot), and a counterfeit banknote is still a banknote, of sorts, isn't it? 

So we need to think of these kinds of adjectives as being subsectives, really. Or if you prefer, the noun encompasses all the things that are putatively or ostensibly that thing: detective includes real and fictional ones, as well as impostors who pretend to be detectives to scam you. Banknote includes not just legal tender but fake notes put into circulation by counterfeiters. And leather includes not just the kind made of animal hide, but includes imitation leathers as well. 

Perhaps more accurately, we would want to say that in different contexts leather has both a broad meaning and a narrow or strict meaning (this is why you get words like SALAD-salad, to indicate the narrow sense). Then we have to step away from semantics into the realm of pragmatics and communication, and note that the customers for whom it's most relevant to know that the item is made of PU leather are vegans, and for them it's actually not helpful to use this broad definition of leather in the description, because they would buy products made only from the subset of imitation leather.