  According to Cockburn, fitness theory assumes that both human nature and God’s creative will are fundamental for morality. Fitness theory rests upon the view that human interest is a basic expression of a human nature brought into being by divine will. Natural good and evil are thus fundamental concepts for moral fitness theory, for it is human nature and all that is associated with it that provides the basis for moral law. For Cockburn, it is through knowledge of our own and God’s natures that humans can understand the right and proper relations that ought to subsist in the universe. According to this account, moral obligation arises from the demands of our natures – that is, we ought to be guided by that which is suitable and proper to our natures as rational and social beings. Common sense judges virtuous practice according to what is fit and natural, and not merely according to the good the practice produces.