What were the parts of the psyche posited by Plato in the Republic? The question calls not for a list but for an account of the grounds on which Plato posited and distinguished such parts. It calls for an account of their nature and role in his system.
We are not lacking in attempts at such an account. These parts have been termed "faculties", "principles", "activities", "aspects", "instances", and "levels" of the psyche. Their nature and role have been characterized in ways influenced as much perhaps by the connotations of these terms as by the details of Plato's text. Yet scholars have long known better. Ritter, Taylor and Graeser, to name only three, have plainly warned of the confusions that result from characterizing the parts Plato posits as if they were psychological divisions more familiar to us.
Unhappily, to warn of possible confusions is not sufficient to deliver us from them, for they continue to be encouraged by familiar works of interpretation. Perhaps the most common of these confusions results from the suggestion that the parts of the psyche posited by Plato were facilities ~ "powers" or "capacities" on the order of those posited by Aristotle and modern psychologists. It will be helpful in dispelling this and other such confusions to ask the question, "What were the parts of the psyche posited by Plato?", and to minimize the seductive influence of any later psychological terminology in answering it. The answer can then be applied to illuminate related themes in later dialogues, especially the Phaedrus.
Moline, John, "Plato on the Complexity of the Psyche" (1976). The Society for Ancient Greek Philosophy Newsletter. 279.