In the _Spirit of Laws_ to which reference was made in Chapter XI, Montesquieu accentuated the importance of environmental influences on social processes. He attempted to show the effects of climate upon social institutions. Montesquieu did important pioneer work in what is now known as the field of anthropo-geography. By way of contrast, the attitude of Hume, whose contributions to social psychology have already been noted, stands out sharply. According to Hume, physical causes have no particular effect on the human mind. No geographic factors influence either the temperament, disposition, or ability of people. Hume was led to this extreme position by his staunch faith in the subjective and psychological factors of human nature. The distinguished German scientist, Alexander von Humboldt (1769–1859), travelled extensively throughout the world, observing the physical geography of many lands in conjunction with the meteorological conditions of each. At the same time von Humboldt was a careful observer of the customs, manners, and standards of the various peoples with whom he came in contact. In these travels and studies, von Humboldt was careful to note relationships between soils and civilizations. His contributions to social thought were of this descriptive nature, based on first-hand observations in many parts of the world. The writings of Henry Thomas Buckle (1821–1862) contain an extensive and detailed explanation of the ways in which geographic and natural factors modify human life. Buckle starts with a decidedly dualistic universe--a dualism which is disjunctive. The dualism consists of nature and mind, each subject more or less to its own laws. Rejecting both the doctrine of free will and of predestination, Buckle concludes that the actions of men are determined solely by their antecedents and that they have a character of uniformity.[XV-2] Man modifies nature, and nature modifies man, but in the past in many parts of the world the thoughts and desires of men are more influenced by physical phenomena than they influence such phenomena. Because of this dominant activity of the physical forces, these should be studied as a basis for understanding the history of man. The physical factors which have powerfully influenced men are four: climate, food, soil, and the general aspects of nature. By the fourth, Buckle refers to those appearances which are presented chiefly through the medium of sight and which produce their chief results by exciting the imagination and suggesting superstitions. The three first-mentioned factors do not operate on the mind directly.[XV-3]