People can cope fine with hierarchies when different things go at different levels of the hierarchy. Recursive hierarchies? A bit harder. So: God at the top, t

People can cope fine with hierarchies when different things go at different levels of the hierarchy. Recursive hierarchies? A bit harder. So: God at the top, then the king, then the barons/lords/etc until you get to the peasants at the bottom. Easy to understand. Or: shopping centre at the top, then different floors in the shopping centre, then various shops, then aisles in the shop, then vaguely demarcated lengths of aisle, then shelves, then individual items on shelves. Fine. But: a folder at the top, then more folders in that folder, and further folders in those folders? And all those folders look the same (only the names are different. Actually, sometimes the names are the same as well). That is a bit confusing for most people. Also consider menus. A menu bar across the top, with a single level of drop down menus underneath? Easy peasy. A start button with a pop up menu, that contains more pop up menus, that contain still more pop up menus? Disorienting. Microsoft thought the only thing holding back deep hierarchical menus was mouse control, so when they refined that for Windows 95 they went crazy with menus inside menus inside menus. Mistake. They've been trying to flatten and differentiate different levels of the start menu ever since. Meanwhile, programmers need to be good at abstraction, so have no problem with undifferentiated folders within folders within folders.