By saying a "set of", you are referring to multiple items. "custom" is just one, whereas "customs" refers to multiple/a set of things. By making it plural, you are making more of a general statement. If you wanted to use the non-plural way, you could say: "He sees it edited by a definite custom, institution, and way of thinking." In this example you would be referring to one specific "custom, institution, and way of thinking" rather than a generalization of it all. Hope this helps

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!