Final answer:
John Dickinson believed that the American colonists should oppose the Townshend Duties because he argued that Parliament did not have the authority to impose internal and external taxes without their consent.
Step-by-step explanation:
In his influential pamphlet 'Letters from a Pennsylvania Farmer,' John Dickinson argued that American colonists should oppose the Townshend Duties imposed by the British Parliament. He believed that while the Empire had the right to regulate trade, Parliament did not have the authority to impose internal taxes or external taxes on imports without the colonists' consent. Dickinson's argument was based on the idea that taxation without representation was unjust and infringed upon the colonists' rights as British subjects.