The answer is Enjambment, because an enjambment has incomplete syntaxes at the end of a line, the meaning runs over from one line to the next and is end-stopped. A caesura is always physically represented in poetry with "||" and it is used to show a literal pause between whatever is being said (you can hear a caesura if you ever listen to William Shatner speak). Pope's "An Essay on Man" follows actual poetry form and therefore cannot be blank verse, open form, or free verse. It is easier to see enjambment when the poem is written out in the correct visual format:
Know then thyself, presume not God to scan
The proper study of Mankind is Man.
Placed on this isthmus of a middle state,
A Being darkly wise, and rudely great:
With too much knowledge for the Sceptic side,
With too much weakness for the Stoic's pride,
He hangs between; in doubt to act, or rest;...
