How do I find key themes?

To find key themes in poetry and prose you should revise the content of the text in order to group similar events or similar words. For example, you would find a lexical set of greed in Seamus Heaney's 'Blackberry picking' with words such as "sweet", "hunger" and "glutting" denoting that greed is one of the key themes. If you continue to find these groups of similarity you should soon have a list of key themes.