I think the middle class in any society comprises those people in charge of decision making. Now, "decision making" or "decision makers" if you like, is quite ambiguous. Precisely put, I mean the people placed in such positions as to be able to influence the economic and political dynamics. The determinant here is one's purchasing power as stated by Matthijs van den Broek above. Once one is able to consume to his substantial satisfaction (i.e meeting all life's basics) then I think they qualify to fall among the middle class. But this is not entirely all--for there is the question of status in society. But this is easily answered by one's ability to purchase and consume goods and services, hence the nexus between economics and politics. Knowledge and access to information determine status and this is easily attained once one is able to purchase commodities.