hmm....... To switch..?

Are The Grounds a group of people, or a family? I think it could be written again like this: "The Grounds were all handsome, blue-eyed, fair-haired, clever and ambitious" or " All The Grounds were handsome, blue-eyed, fair-haired, clever and ambitious"

I think it's because "the world" is broken up into, or "edited" by, so many different cultural traditions and ways of thinking. Not just one. There is a "definite set" of them, certainly, but a multitude of traditions and institutions and customs within that definite set. People believe in all sorts of conflicting religions and philosophies; national customs and institutions based on various systems, and alternative ways of thinking. These come together to make up humanity, but they can all be defined in their own right. Therefore it would not be correct to use the singular "custom" or "institution" or "way" in this instance. It needs to be plural because there are many of them, but only one definite set. Another way to look at it would be through animals. There are millions of different kinds of animals, but 'animal' is a definite set that can be broken down further into different species of animal, like dogs for example. Dogs can be broken down again into different breeds of dogs. And further again to individual dogs. All these species and breeds and individuals are plural because there are many of them, but they all fall under the definite set of animal, which is singular. I hope that helps! Interesting question!