“Now I want you to tell me truly whether virtue is one whole, of which justice and temperance and holiness are parts; or whether all these are only the names of one and the same thing: that is the doubt which still lingers in my mind.”
Socrates challenges Protagoras to clarify whether moral excellences such as justice, temperance and holiness make up different parts of a single virtue or name the same property. The question tests the structure of virtue rather than giving the reader an easy definition. Protagoras proposes an analogy with distinct parts of a face, and their discussion goes on to scrutinize the claim.
Source details
Plato, Protagoras, approximately Stephanus 329c–d; Socrates asks Protagoras about the unity of virtue after the sophist describes civic instruction and the acquisition of good character. Protagoras, not Socrates, answers that its qualities are distinct parts. Exact sentence from Benjamin Jowett English, Project Gutenberg #1591, in the dialogue proper after the opening philosophical and mythic arguments, not in translator commentary.