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."