Human technology is difficult to understand because it is so complex. However, human technology evolved from the simpler technologies of other species. Comparison with these other technologies should illuminate why human technology is distinct. Some birds and primates make tools, or simple technological objects whose function is closely related to their form. Humans, on the other hand, make machines-relatively complex objects whose functionality derives from the interaction of parts with respect to one another (e.g. a bow and arrow). Making machines requires a cognitive advance called 'second-order instrumentality', or the ability to invest in the production of an object that only has utility as part of, or for the making of, other objects. This ability enabled human societies to develop specialised forms of organised production, which in turn allowed the stock of artefacts to diversify and accumulate, whereas the technological repertoires of other species remain at a relatively constant level of complexity. © The Author 2009. Published by Oxford University Press on behalf of the Cambridge Political Economy Society. All rights reserved.