To be patented, your invention not only must fall within one of the three statutory classes, it also must fulfill three other requirements:
Novelty
Your invention must be new. If it has been publicly known, used or sold anywhere in the U.S., or described in printed material available anywhere in the world before the date of your patent application, your invention cannot be patented. A note of caution: Even if you yourself use the invention publicly or describe it on e-mail, at a conference or in published form more than a year before your patent application, you've likely disqualified your invention from receiving a patent. Furthermore, secret efforts toward commercialization by an inventor can constitute a disclosure that can disqualify a patent.
Utility
Being new isn't enough nor is being different. For your invention to be patentable, it also must be useful. The invention must perform a function, do what you say it does, and benefit society in some way. A machine that doesn't perform its intended purpose is not useful, and therefore cannot be patented.
Nonobvious
To be patentable, your invention must give new and nonobvious results compared to known approaches. Ordinary differences in size, materials or other obvious modifications are generally not patentable.
Determining patentability is a complicated undertaking. A reputable patent attorney or agent can be consulted to sort out the legal and technical complexities.
The Inventor Handbook is written as a preliminary guide for inventors and where legal rights are concerned, the reader should consult a qualified attorney.
