Weber recognized explicitly that his concept of ideal-type is directly borrowed from economic theory and as it is commonly admitted from the German-speaking ‘marginalist school’. Nevertheless, the construction of ideal-types reminds greatly the definition of economic rationality made by John Stuart Mill, who also built up a concept to explain, in individualistic terms, the real world in a given historical and geographical context. The position defended here is that Weber generalizes Mill's methodological proposition of concept formation regarding economic rationality to accomplish his much larger project of determining the social factors responsible for the rationalization of the Western civilization.