Stage theory holds that each field of science undergoes an unstable stage and then a stable stage. In the unstable stage, it is likely that scientists learn through trials and errors, and that an existing theory is false. In the stable stage, by contrast, it is likely that scientists no longer learn through trials and errors, and that an existing theory is true. According to stage theory, past scientists accepted a past theory because they were in an unstable stage, and current scientists accept a current theory because they are in a stable stage. This account is better than two pessimist accounts of why past scientists accepted a past theory and why current scientists accept a current theory.