This is a very old book of poems dating from 1744. The four books are four aspects of what is needed to create and maintain health: Air; Diet; Exercise; The Passions. The first three are straightforward. In 'the passions' Armstrong is really talking about mental health and describes it thus, “ it now remains to trace What good what evil from ourselves proceeds: And how the subtle principle within Inspires with health, or mines with strange decay The passive body.”