Plato claims in the Republic that the philosopher should rule in the ideal city. The philosopher, as defined here, is a person who has the knowledge of the Good and is just on the basis of this knowledge. But there are some passages in the Republic that seem to imply that anyone who does not have this knowledge cannot be just at all. This implication is counterintuitive, and I argue that such thought goes against what Plato really believes. I begin with an analogy of violin-making and explain, with the use of this analogy, the differences among the wisdom-lover, the honor-lover, and the money-lover in Republic IX. I distinguish two kinds of each lover: one who has the ability to achieve by him/herself what he/she loves, and one who does not have that ability. With this distinction we can see that there can be a wisdom-lover who lacks the ability to achieve the knowledge of the Good by him/herself but still pursues virtue and justice for its own sake. I then pay attention to subtle differences between the descriptions of the guardian-ruler in Republic II-IV and those of the philosopher-ruler in Republic V-VII, and show that the conditions for being the former are logically different from the conditions for being the latter. I argue on the basis of these logical differences that it is not absolutely necessary that the just man in Republic IV have the knowledge of the Good; he may only have some sort of true belief about goodness. And a just man who has only true belief is a non-philosopher wisdom-lover.