David Livingstone stresses "that geography has meant different things at different times and in different places" and that we should keep an open mind in regards to the relationship between geography and colonialism instead of identifying boundaries. Geography as a discipline was not and is not an objective science, Painter and Jeffrey argue, rather it is based on assumptions about the physical world. Comparison of representations of ostensibly tropical environments in science fiction art support this conjecture, finding the notion of the tropics to be an artificial collection of ideas and beliefs that are independent of geography.