Things that cannot be patented include:
- Laws of nature
- Physical phenomena
- Abstract ideas
- Literary, dramatic, musical, and artistic works (though these can be Copyright protected)
- Inventions which are not useful (such as perpetual motion machines); or are offensive to public morality.
In order to be eligible for a patent, inventions must also be:
- Novel
- Nonobvious
- Adequately described or enabled (for one of ordinary skill in the art to make and use the invention)
- Claimed by the inventor in clear and definite terms
(Information from the USPTO "Patents for Inventors" webpage.)