Ideas Informing Ideas vs Theft

If art informs art, can we not say ideas inform ideas? Okay, but when is this the process that's happening, and when is an idea/thought being plagiarized? This conversation covers many facets. Is plagiarism illegal? What would you do, if you were copied? Would it bother you? If it would, is it because we feel we own the thought/idea? Why is this such a threat? How different listeners would approach such a situation. Plus much more.