Evidence is offered to support two claims about human metacognition. First, metacognition is a ubiquitous part of cognition and has deep evolutionary roots. It is claimed that two driving forces for the evolution of metacognition are: awareness that the senses are error prone, and the competing objectives involved in carrying out even a simple task. Reasoning and metareasoning are needed to select among behaviors that on one hand have outcomes that depend on the situation estimate, and on the other hand have different impacts on each of the competing objectives. Second, metacognition has two important components — metareasoning within a single reasoning frame, and switching between reasoning frames. Fauconnier’s linguistic studies suggest that Mental Spaces act as frames in which humans can do reasoning. Fauconnier has studied the construction and initialization of Mental Spaces extensively, but has been less specific about the process of switching between Mental Spaces. A case is presented for emotions playing a critical role in this process by triggering and directing the switches between Mental Spaces. Affective cognitive models that can already perform emotional computations can readily be adapted to direct switching between reasoning frames.