Defoe says that there were a hundred thousand country fellows in his time ready to fight to the death against popery, without knowing whether popery was a man or a horse.

In this quote, William Hazlitt, an English essayist and critic, refers to a statement made by the writer Daniel Defoe about the ignorance of the common people in relation to the political and religious conflicts of their time. Hazlitt is quoting Defoe’s remark that there were many ordinary individuals, or country fellows, who were willing to fight and die to oppose popery, a term used to describe Catholicism, without truly understanding what it even meant. The phrase "without knowing whether popery was a man or a horse" humorously emphasizes the ignorance and blind loyalty of these people, who were eager to support a cause without any real knowledge of what they were fighting against.

Hazlitt uses this quote to illustrate how people can be easily swayed into action or conflict without understanding the true nature of the issues at hand. The idea that individuals were ready to fight without knowing the actual meaning of popery serves as a critique of fanaticism and the manipulation of the masses by political or religious leaders. Hazlitt's point is that these people were willing to engage in violence and war simply because they were told to, without critically questioning or understanding the cause they were being asked to fight for.

The quote also speaks to the broader theme of irrationality in human behavior, particularly in political and religious matters. Hazlitt critiques how individuals are often motivated by fear or prejudice rather than knowledge or reason. The example of popery represents a time when religious conflicts were deeply ingrained in society, and people could be easily manipulated into extreme actions, such as violence or war, based on ignorance or misinformation.

Ultimately, William Hazlitt uses Defoe’s observation to expose the dangers of blind fanaticism and the ease with which the masses can be mobilized to support causes they don't fully understand. The quote serves as a warning about the importance of critical thinking and understanding before engaging in potentially destructive actions, especially in the realms of politics and religion.