Callicott traces the conceptual foundations of the Leopold land ethic first back to Charles Darwin’s analysis of the “moral sense” in the "Descent of Man" and ultimately to David Hume’s grounding of ethics in the “moral sentiments” espoused in "An Enquiry Concerning the Principles of Morals". Hume argues that moral actions and moral judgments are based on such other-oriented sentiments as sympathy, beneficence, loyalty, and patriotism. Darwin argues that these “moral sentiments” evolved as the "sine qua non" of social (or communal) solidarity, on which depends the survival and reproductive success of the individual members of society (or community). The tradition of dichotomous thinking in Western philosophy inclines most philosophers to dismiss Hume's ethics as a kind of irrational emotivism, despite the fact that, Callicott believes, Hume clearly provides a key role for reason in moral action and judgment. The faculty of reason, according to Hume, determines (1) relations of ideas, which are essentially logical relationships; and (2) matters of fact. Among such matters of fact, reason both traces the often complex causal chain of the consequences of various actions and discloses the proper objects of the moral sentiments. Accordingly, Leopold also traces both the causal chain of ecological consequences of such seemingly innocent actions as tilling the soil and grazing cattle and discloses a proper object of those moral sentiments — such as loyalty and patriotism — which are excited by social membership and community identity. That proper object of such sentiments is the “biotic community,” revealed by the relatively new science of ecology.