He does not dismiss externally oriented explanations of profitability,
but wishes to explore the internally oriented explanation that idiosyncratic firm resources are at the base of superior performance.
He sets out on this task by pinpointing the two fundamental assumptions on which the resource-based view rests – that firms have different resources (resource heterogeneity) and that these resources cannot be easily transferred to, or copied by, other firms (resource immobility). He goes on to argue that these resources can be the basis of competitive advantage if they meet four criteria: they must be valuable and rare, while being difficult to imitate and substitute.