Alternative medicine refers to any practice or treatment that aims to heal, but lacks the biological plausibility of traditional (mainstream) therapies. Alternative therapies reside outside medical science. They may be based on things such as religion, tradition, hearsay, superstition, belief in supernatural energies, pseudoscience, etc. and are thus quite controversial at times. Many believe that alternative medicine to be untested, untestable, fraudulent or even proven ineffective, and are thus skeptical of trying it out.