A theory of human nature is a descriptive theory that ascribes certain general properties or features to human beings. Therefore, it may err by including features that people do not possess (e.g., perfect wisdom and perfect benevolence) or by not taking into account features that people do possess. But a theory of human nature need not include all features that most people possess—this would yield a long and relatively useless list. In fact, the theory must select a certain subset of these features, but which subset? An initial proposal is that as a theory of human nature it must select properties that are possessed by all (or nearly all) human beings. The combination of features selected must be possessed by humans alone, or else we would have a theory of the nature of a wider class of beings. Also, the properties in question must be very difficult or impossible to alter by changing the natural or social environment, or else they cannot plausibly be attributed to our natures. In sum, a theory of human nature picks out those features that are unalterably possessed by (nearly) all human beings and are together possessed by them alone.

(Gregory S. Kavka, Hobbesian Moral and Political Theory [Princeton: Princeton University Press, 1986], 29-30 [italics in original; footnote omitted])