For an object or idea to be patentable, it must fit into one of the following four categories; process, manufacture, machine or composition of matter. The subject matter must also be original, an unmodified, previously existing invention is never patentable. There must be a significant improvement over previous inventions for the new one to qualify. If two previous inventions are combined together, the combination must yield new and unexpected results for the invention to be considered patentable.

In addition, an invention must be useful and must actually work in order for it to be patentable. A useful invention is one in which the object already has a utility without anyone having to pursue further research to identify or reasonably confirm the utility. So, if you've invented a nifty little widget or doodad, but haven't got a clue as to what it could be used for, the PTO isn't going to be impressed and isn't likely to grant your patent.

On the bright side, if an invention does not accomplish all of its intended functions or it only has partial success, it may still be patentable. In the case of newly developed drugs, the claimed invention only needs to treat a single symptom of an incurable disease for it to have usefulness. The Patent Office isn't as strict on drugs and treatments (that's where the Food and Drug Administration come in).

The PTO has established that laws or forces of nature are not deemed patentable. Examples of these include, but are not limited to, the law of gravity or E=mc2. Computer related inventions may or may not be patentable. Computer programs that have a function when used with a computer are definitely patentable subject matter. Merely recording information (like music, literary works or data) on a computer-readable medium will not result in a patentable idea.