Justice Potter Stewart is famous for ruling on an obscenity case by saying, “I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description[hard-core pornography]; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.” Sometimes he was critisized for being vague and setting a subjective bar, but also praised for practically recognizing the difficulties of defining this sort of thing.
But, let’s actually talk a bit about an implicit assumption in what he says, which is that if you come up with a list of necessary and sufficient conditions for membership in a category, you would know what falls into that category. That is our lay-person definition of a category, and how things fall into that category. However, there are a number of problems with this “Classical Theory of Categorization”. It, incidently, comes from Plato and Aristotle like many things, and basically defined something as a “duck” if it was a bird and floated in water. But that definition is too broad and can include geese and other birds, so define it further, a duck has a short neck, but so does other birds that float in water. And so on. You can apply the same process to figuring out what constitutes as obscenity and pornography and what might constitute art, for example.
A philospher by the name of Wittgenstein eventually realized that any definition for a category dies the death of a thousand counter-examples. Later on, a psychologist named Eleanor Rosch was inspired by him and strove to create a theory of categorization that would be more accurate, which in this case was the Prototype Theory. Prototype theory basically states that things are compared a (usually non-existant) prototype and thus can be rated as being typical of a category or not. For example, a sparrow is a more typical of the category “bird” than a penguin or an ostrich.
In a basic way, Judge Stewart was on the money when he chose to reject the definition-based model and instead choose a subjective model, which indeed tells him better what hard-core porn would be than a definition-based approach, or, whether an emu is a bird or not, for that matter.