The use of the first, second or third person when writing any literary, whether it be fictional or non-fictional, work, depends on the purpose of the writer and the objective that the work has. However, there are differences between academic and non-academic writing that will allow a writer the freedom of using or not using the first, second or third person to express views.
First, the use of the first person has the purpose of allowing the writer to show his/her audience his/her personal views on a matter. This type of voice allows the reader to have a closer experience and a front-row seat to the thoughts of the writer. However, the problem, especially when writing academic materials, is that the first person is highly subjective and academic writing, especially essays, depend, to be taken seriously, on the way the writer exposes his/her ideas not as her personal views, but as the general views.
On the other hand, the third person is a type of voice that gives objectivity to writing, making the writer seem like he/she is expressing strictly verifiable facts and data and not merely what comes from his/her thoughts. The problem with only using this voice is that, as said, it is strictly academic and does not give much room for the reader to have a closer experience with the writer and the work.
This is why the use of both voices, first and third, enhances the essay of Cooper; because it allows the reader to have both experiences at the same time. First, have a closer feel to the thoughts and feelings of the author, and second, know that there is also objectivity in the work. Cooper does not merely express her beliefs and ideas without any proof, but she shows that she uses and speaks of verifiable fact, but in a more informal and easy-to-read type of wiriting.