according to Madison, what causes faction?

Madison defines factions as groups of people who gather together to protect and promote their special economic interests and political opinions. Although these factions are at odds with each other, they frequently work against the public interest, and infringe upon the rights of others.

Both supporters and opponents of the plan are concerned with the political instability produced by rival factions. The state governments have not succeeded in solving this problem; in fact, the situation is so problematic that people are disillusioned with all politicians and blame government for their problems. Consequently, a form of popular government that can deal successfully with this problem has a great deal to recommend it.

Given the nature of man, factions are inevitable. As long as men hold different opinions, have different amounts of wealth, and own different amount of property, they will continue to fraternize with people who are most similar to them.