In book four of Plato's “The Republic,” Socrates defines justice in the individual as analogous to justice in the state. I will explain Socrates' definition of justice in the individual, and then show that Socrates cannot certify that his definition of justice is correct, without asking further questions about justice. I will argue that if we act according to this definition of justice, then we do not know when we are acting justly. Since neither the meaning of justice nor that of good judgment is contained in the definition, then one can act unjustly while obeying the definition of justice. If one can act unjustly and still obey this definition, then Socrates' definition of justice is uncertifiable. Socrates defines justice in the individual as the three parts of the soul doing their work and only their work (433b). The parts of the soul are reason, appetite, and spirit (435c). Reason is the part that thinks and makes judgments. Appetite is the impulsive part that acts on bodily desires. The mind is the emotional part and acts on feelings, like courage. (...