Laws of nature, natural phenomena and abstract ideas are not patentable. Thus Albert Einstein could not have patented his theory of relativity since it is a law of nature.
In order to patent an invention it must pass four tests:

1. First, there are five “statutory classes” of things that are patentable and an invention MUST fall into one of these:
   • Processes
   • Machines
   • Manufactured items (objects made by humans or machines)
   • Compositions of matter
   • New uses of any of the above
2. The invention must be “useful”. One aspect of the “ utility” test is that the invention cannot be a mere theoretical phenomenon.
    
3. The invention must be “novel”, that is, it must be something that no one had done before.
    
4. The invention must be “non-obvious” to “ a person having ordinary skill in the art to which said
   subject matter pertains”. This requirement is the one on which many patentability disputes
   hinge.