Humans construct theories in order to explain, predict and master phenomena (e.g. inanimate things, events, or the behavior of animals). In many instances, we are constructing models of reality. A theory generalizes about observations and consists of an interrelated, coherent set of ideas and models. According to Stephen Hawking in A Brief History of Time, "a theory is a good theory if it satisfies two requirements: It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations". He goes on to state, "any physical theory is always provisional, in the sense that it is only a hypothesis; you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single repeatable observation that disagrees with the predictions of the theory". This is a view shared by Isaac Asimov. In Understanding Physics, Asimov spoke of theories as "arguments" where one deduces a "scheme" or model. Arguments or theories always begin with some premises - "arbitrary elements" as Hawking calls them (see above), which are here described as "assumptions". An assumption according to Asimov is "something accepted without proof, and it is incorrect to speak of an assumption as either true or false, since there is no way of proving it to be either (If there were, it would no longer be an assumption). It is better to consider assumptions as either useful or useless, depending on whether deductions made from them corresponded to reality.... On the other hand, it seems obvious that assumptions are the weak points in any argument, as they must be accepted on faith in a philosophy of science that prides itself on its rationalism. Since we must start somewhere, we must have assumptions, but at least let us have as few assumptions as possible."