act

The root act comes from Latin agere, one of the busiest verbs in the entire language. Agere meant "to do, to drive, to set in motion" — the same word a Roman farmer used for driving cattle, a general for leading an army, and a lawyer for pleading a case. Latin verbs leave behind two stems: the present stem (ag-, seen in agere) and the past-participle stem (act-, from actus, "done, driven"). English inherited both, and act- became the more visible of the two.

From actus came the simplest words first: act (a thing done, or to do something) and action (the doing itself). Add a suffix and you get the people and qualities: an actor acts, an activist acts on convictions, activity is the state of acting, and to activate is to make something start acting.

The real power of act shows up when prefixes give the action a direction:

- re- (back) + act → react: act back at something — a response
- inter- (between) + act → interact: act between each other
- counter- (against) + act → counteract: act against, to cancel out
- en- (make) + act → enact: make something into action — pass a law
- trans- (across) + act → transaction: business carried across between two parties
- retro- (backward) + active → retroactive: an action reaching backward in time

One member hides its origin completely: exact comes from ex- (out) + agere — literally "to drive out" or "weigh out." A Roman tax collector exacted payment by driving it out of you, and to weigh something out precisely is to be exact. The "precise" meaning and the "demand forcibly" meaning are two faces of the same act of driving something out to the exact amount.

Notice the pattern: act stays constant as the core idea of doing, and the prefix tells you which way the doing points — back (react), between (interact), against (counteract), or out (exact). The twin stem ag- (agent, agenda, agile) carries the same verb in its present form; the two are really one Latin word wearing two coats.