The notion of Human Nature is often called upon to carry the burden of a great number of arguments: (i) All men, we are told, are motivated to action by the same basic desires (needs, instincts, drives). Unity is given to a great variety of himian behaviour if we see them as emanating from a single source. (ii) Given that human nature is what it is, then social institutions and political organisations ought to be such and such. Human nature is the one unchangeable constant in a varying social system. (iii) Given that human nature describes certain basic and common needs, then the 'good man' is a man that has had these needs satisfied. (Just as a good plant is one that has had its needs satisfied). Good and bad are determined in terms of the basic human requirements. Theories of human nature, therefore, have powerful consequences. It is important that one should be clear about what exactly is involved in this notion of 'a human nature'. Statements that assert something about the nature or essence of man are by no means restricted to philosophers. We have on the one hand the traditional, and largely speculative claims, of Plato, Hobbes, and Rousseau. But we have also the more scientific claims of psychologists (Freud, Maslow, Fromm) and anthropologists (Lorenz, Ardrey, Morris). Psychologists and anthropologists claim new insights for why we do the things that we do. They give explanations of hujnan behaviour in terms of general and basic laws. These generalisations turn out to be descriptions of human nature. I argue that the scientific enterprise of formulating such general laws for the purpose of explaining human action is radically misconceived. How does the human nature theorist arrive at his theory? That is, how is one supposed to make this discovery about a human essence? what do we look for; where do we look for it; and how do we go about discovering it? These are important questions that need to be asked of both the psychologist and the anthropologist. They will lead us to both a clearer understanding of the kind of argument involved in their claims, and will also bring to the forefront the underlying assumptions in their position. Human nature theorists employ a number of terms that seem to be essential to the theories. Terms like 'desire', 'need', 'drive', and 'instinct' function as 'primary elements' because any formulation of a general law that purports to have explanatory force with regards to hi.iman behaviour must employ terms that are generally and ordinarily employed to explain particular actions. Explanation of human behaviour by reference to the general law achieves a certain plausibility by giving the impression of doing what we ordinarily do when we explain another's behaviour, while at the same time claiming scientific status for the explanation. Concentrating, therefore, in particular on the psychologist's use of 'need' (viz. Maslow: 'all men need love'), and the anthropologist's use of 'instinct' ( viz.Ardrey and Morris: 'all men are born with the instinct for aggression'), I attempt to show that these scientific explanations of human behaviour are incoherent for they depend on the 'ordinary' use of these terms to achieve plausibility while at the same time they depend on a 'scientific' use to legitimise their conclusions.