The term is often used colloquially to refer to any explanatory thought, even fanciful or speculative ones, but in scholarly use it is reserved for ideas which meet baseline requirements about the kinds of observations made, the methods of classification used, and the consistency of the theory in its application among members of that class. These requirements vary across different fields of knowledge, but in general theories are expected to be functional and parsimonious: i.e. a theory should be the simplest possible tool that can be used to effectively address the given class of phenomena.