Purpose and Main Idea actually confuse a good deal of students, perhaps because they are so closely related.

Generally, the main idea of a passage is what the passage sets out to discuss.

The author's purpose is why the author set out to discuss that idea.

One of the easiest tools I've come to use in distinguishing the main idea of a passage from the author's purpose is using an action verb (defend, explain, counter...). Let's use the paragraph below as an example:

The recent assignment of viruses into the taxonomic domain Acytota (non-cellular life) implies that viruses are living organisms, but it would be easy to make too much of this suggestion. The debate about whether or not viruses live is heavily weighted to one side: most scientists have concluded that viruses are nonliving.

This paragraph is almost certainly concerned with the debate over classification of viral life. We can be assured the author will outline both sides of this debate using some detailed (and technical) examples from scientific discovery.

The author's purpose seems slightly different; to truly understand purpose, we need to get a grasp on the author's tone. The author's voice should be evident when the passage states "but it would be easy to make too much of this suggestion'. The author seems to take some issue with the new classification of viruses as non-cellular life. Therefore the purpose seems to be to analyze (or even question) the new scientific classification of viral life.

Main Idea: The debate surrounding classification of viral life.

Purpose: To analyze the debate surrounding that classification.

In the end, thinking through the purpose is very useful because it forces you to examine the author's voice (and that is perhaps the most important thing to take away from RC).[/i]