Projects are linear. There they happen in time. Categories are abstract. They're sort of up in the air. And because your work is best defined in terms of projects, you want to also organize your knowledge according to jects. That's what each one of your project folders is. It's it's a cognitive environment that you can step into and have to think anything else but that one project.
Instead of organizing ideas by category – such as “psychology,” “marketing,” or “economics” – which leads to silos that they can’t escape from, organize your ideas according to the projects where they will be most useful and actionable. Organizing your ideas according to which project they apply to also makes it easier to decide where they go, and easier to find when you need them.