Among the most common informal fallacies in reasoning are fallacies of ambiguity. These are mistakes that hinge on a word or phrase that has one meaning in some or all of the premises of the argument but another meaning in other premises or in the conclusion. Many modern theories describe concepts of individuals or kinds as though these thoughts were reducible to thoughts or judgments about complexes of properties and then ignore the question of what it is to think of a property. Abilities to identify and reidentify appearances of the same objective thing as appearances of the same constitute a substantial part of the possession of any empirical concept. One's rationality depends at every point on the complex causal and informational structure of the empirical world. Rationality is firmly embedded in the world outside the mind.