This examines speculative manias and their relationship to market rationality, exploring how periods of excessive investment in particular assets often disconnect from the theoretical assumption of rational economic behavior. It differentiates between rational expectations, where investors are fully aware of long-term implications, and adaptive expectations, where future values are projected from recent trends, often leading to "greater fool theory" scenarios. The text illustrates how displacements—external shocks like wars, financial innovations, or political changes—can initiate these manias, which frequently unfold in two stages: an initial rational response followed by a surge driven by anticipated capital gains, often involving distinct insider and outsider speculators. Ultimately, the sources suggest that even if individual actors behave rationally, the collective market can exhibit irrationality due to factors like mob psychology or the fallacy of composition, challenging the notion of perfectly efficient markets.