In this critique of Plato and his ideal state (Utopia), the essence of Will Durant's criticism can be summarized as follows:

1. Lack of appreciation for change and flux: Durant argues that Plato's philosophy and his ideal state emphasize order and stability at the expense of recognizing the natural flux and change inherent in the world. Plato's love for order makes him hesitant to embrace change and innovation.

2. Neglect of individual values: Durant suggests that Plato's focus on societal organization and categorization of people into classes overlooks the importance of individual values and personal liberties. His philosophical approach tends to subdue the significance of the individual.

3. Classification of people: Durant compares Plato to an entomologist classifying flies, implying that Plato's categorization of citizens into rigid classes lacks the nuance and diversity that should be present in a just and ideal society.

4. Potential for stagnation: Durant contends that Plato's static state could easily become a society dominated by inflexible elderly leaders resistant to new ideas and changes. This could result in a stagnant society that hinders progress and innovation.

5. Neglect of liberty and art: According to Durant, Plato's state places a high value on order and beauty but does so at the expense of individual liberty and the role of artists in creating and appreciating beauty. The state may prioritize aesthetic ideals but might suppress the creative freedom necessary for true artistic expression.

6. Comparison to Sparta or Prussia: Durant characterizes Plato's ideal state as resembling Sparta or Prussia, which were known for their strict, authoritarian, and conformist governance, rather than an ideal utopian society.

Durant's criticism of Plato's Utopia centers on the potential for a static, authoritarian, and inflexible society that neglects individual values, personal liberty, and the role of artists. He argues that Plato's obsession with order and beauty could lead to a society that lacks the vibrancy and dynamism required for a truly ideal state.