This is a list of substances or materials generally considered discredited. A substance can be discredited in one of three ways: 1. * It was widely believed to exist at one time but no longer is. Such substances are often part of an obsolete scientific theory. 2. * It was once believed to have drastically different properties from those accepted now. It was widely claimed and believed to possess significant properties that are no longer attributed to it. 3. * It is currently believed to exist as part of a theory that has not met the theoretical and experimental requirements of mainstream science. In particular, such a theory must be predictive.