Realistic human rationality departs from ideal theories of rationality and meaning developed in epistemology and economics because in human life deliberation takes time and effort, ignorance and inconsistency do not deter action, and learning takes time and slows with time. This paper examines some theories of mental change with an eye to assessing their adequacy for characterizing realistic limits on change and uses a simple kind of reasoning system from artificial intelligence to illustrate how mechanical concepts, including mental inertia, force, work, and constitutional elasticity, provide a new language and formal framework for analyzing and specifying limits on cognitive systems.