what was the belief of Greek philosopher Plato that music can directly influence a person's character called?

The Theory of Forms

Explanation:

However, we have to understand better how music is introduced in this theory. According to Plato, the world is divided into two forms - the physical and the ideas. He believed that only the world of the ideas was perfect because didn't change. According to his theory, art imitates physical things, such as landscape painting; the song of the bird; someone's speech, in short, all these would be appearances. These, in turn, mimic the pure forms or ideal forms. That's why music can influence the character because goes straight to the mind bringing elements from the physical world and turning them into sensations.