An invention must, in general, fulfill the following conditions to be protected by a patent. It must be of practical use; it must show an element of “novelty”, meaning some new characteristic that is not part of the body of existing knowledge in its particular technical field. That body of existing knowledge is called “prior art”. The invention must show an “inventive step” that could not be deduced by a person with average knowledge of the technical field. Its subject matter must be accepted as “patentable” under law. In many countries, scientific theories, mathematical methods, plant or animal varieties, discoveries of natural substances, commercial methods or methods of medical treatment (as opposed to medical products) are not generally patentable.