An Epistle to Dr Arbuthnot is Pope's Apologia pro sua Satura, or “Defence of his Satire”. All previous masters of formal verse satire, including Horace and Juvenal, had written at least one poem in which they explained why they felt it necessary to attack the persons and objects they did, in the way they did. Thus, when Pope's friend and fellow Scriblerian, John Arbuthnot, wrote to him when the former was seriously ill, in July 1734, making it his “Last Request that you continue that noble Disdain and Abhorrence of Vice, which you seem naturally endu'd with, but still with a due regard for your own safety”, Pope seems to have felt that the time had come for him to publish such a poem.
Pope tells the …
Gordon, Ian. "An Epistle to Dr Arbuthnot". The Literary Encyclopedia. First published 30 March 2001
[http://www.litencyc.com/php/sworks.php?rec=true&UID=6677, accessed 21 February 2017.]