This limitation, however, in no way precludes the construction of mathematical theories that formalize large bodies of scientific knowledge. A theory is "underdetermined" (also called "indeterminacy of data to theory") if a rival, inconsistent theory is at least as consistent with the evidence. Underdetermination is an epistemological issue about the relation of evidence to conclusions. A theory that lacks supporting evidence is generally, more properly, referred to as a hypothesis. If a new theory better explains and predicts a phenomenon than an old theory (i.e., it has more explanatory power), we are justified in believing that the newer theory describes reality more correctly. This is called an "intertheoretic reduction" because the terms of the old theory can be reduced to the terms of the new one. For instance, our historical understanding about "sound", "light" and "heat" have been reduced to "wave compressions and rarefactions", "electromagnetic waves", and "molecular kinetic energy", respectively. These terms, which are identified with each other, are called "intertheoretic identities." When an old and new theory are parallel in this way, we can conclude that the new one describes the same reality, only more completely. When a new theory uses new terms that do not reduce to terms of an older theory, but rather replace them because they misrepresent reality, it is called an "intertheoretic elimination." For instance, the obsolete scientific theory that put forward an understanding of heat transfer in terms of the movement of caloric fluid was eliminated when a theory of heat as energy replaced it. Also, the theory that phlogiston is a substance released from burning and rusting material was eliminated with the new understanding of the reactivity of oxygen. Theories are distinct from theorems. A "theorem" is derived deductively from axioms (basic assumptions) according to a formal system of rules, sometimes as an end in itself and sometimes as a first step toward being tested or applied in a concrete situation; theorems are said to be true in the sense that the conclusions of a theorem are logical consequences of the axioms. "Theories" are abstract and conceptual, and are supported or challenged by observations in the world. They are 'rigorously tentative', meaning that they are proposed as true and expected to satisfy careful examination to account for the possibility of faulty inference or incorrect observation. Sometimes theories are incorrect, meaning that an explicit set of observations contradicts some fundamental objection or application of the theory, but more often theories are corrected to conform to new observations, by restricting the class of phenomena the theory applies to or changing the assertions made.