Each provides an untestable hypothesis that, if true, makes something improbable seem quite probable. This reliance on inexplicable motives of the designer makes intelligent design scientifically untestable. Stone's ideas have been dismissed by medical health experts as quackery or untestable. Strong Gaia, Kirchner claimed, was untestable and therefore not scientific. Compensation for static hazards, namely branching signals, can render a circuit untestable using this model. It is also a safeguard against untestable requirements, misunderstandings, and requirements creep. Radcliffe-Brown rejected both of these views because of the untestable nature of historical reconstructions. While a useful-if simple-model, limiting similarity is nearly untestable in reality. This model has been criticized as circular and untestable. However the prototype was untestable because of mechanical problems.