Plato aims to provide objective and absolute solutions to moral issues, his intelligible realm is where these solutions lie. Plato is a realist and views morality as an objective issue which has a right or wrong answer. Morality is about rationality.
Plato’s largest argument for providing objective moral solutions is his theory of the Forms. The forms are only accessible through reasoning and rationality, according to Plato they are part of the intelligible realm. They are objective, unchanging and infallible therefore we can derive and objective morality from reasoning and the Forms, most importantly the form of the good, in which everything in the world derives meaning and purpose. The objectivity of the forms provides the ideal that abstract concepts such as justice, beauty and morality can have an objective right or wrong answer in any circumstance, through their forms. The form of the good is the most important in providing solutions to moral issues as once you know the good, you automatically do the good. Plato implies that once you have knowledge of the form of the good through reasoning, it is impossible to do otherwise, therefore having knowledge of the form of the good leads to carrying out the objective, correct action in a moral issue. Therefore if you do not know the good, you will only carry out the morally wrong actions and you are irrational so you cannot be held morally responsible for these actions. As knowing the good leads to the good, and only philosophers can access the forms, then it follows that only philosophers can know the good and can provide objective correct solutions to moral issues.
The largest criticism Plato faces is what if his theory of the forms, on which he basis his whole philosophy, is not existent? Plato was unable to prove or provide much detail on the forms and what they are, thus many argue against him suggesting that the forms are not