The Fundamental Thesis of Methodological Individualism Is Ideas Held by Individuals Determine Their Group Allegiance, and a Collective No Longer Appears as an Entity Acting of Its Own Accord and on Its Own Initiative

There is no need to add anything to what has already been said by praxeology and economics to justify methodological individualism and to reject the mythology of methodological collectivism. Even the most fanatical advocates of collectivism deal with the actions of individuals while they pretend to deal with the actions of collectives. Statistics does not register events that are happening in or to collectives. It records what happens with individuals forming definite groups. The criterion that determines the constitution of these groups is definite characteristics of the individuals. The first thing that has to be established in speaking of a social entity is the clear definition of what logically justifies counting or not counting an individual as a member of this group. . . .

The rejection of methodological individualism implies the assumption that the behavior of men is directed by some mysterious forces that defy any analysis and description. For if one realizes that what sets action in motion is ideas, one cannot help admitting that these ideas originate in the minds of some individuals and are transmitted to other individuals. But then one has accepted the fundamental thesis of methodological individualism, viz., that it is the ideas held by individuals that determine their group allegiance, and a collective no longer appears as an entity acting of its own accord and on its own initiative.

--Ludwig von Mises, The Ultimate Foundation of Economic Science: An Essay on Method, ed. Bettina Bien Greaves (Indianapolis: Liberty Fund, 2006), 73-74.