Our mind implements thinking shortcuts that influence our decisions and induce into reasoning errors (bias). Cognitive biases influence clinical reasoning and error in medicine. Bias cannot be eliminated, but it is necessary to know them in order to analyze them retrospectively in order to prevent any distorting effects that could negatively impact the diagnostic and therapeutic process. The two systems of thinking, fast thinking and slow thinking, can both cause diagnostic and therapeutic errors; therefore, both systems should be activated in clinical work to reduce bias.