A key finding of Chapter 3 was that human nature was, in Hume's words, ‘constant and uniform in all its operations’. One facet of this uniformity is that human nature is progressive or developmental. Millar can, accordingly, write:
There is, however, in man a disposition and capacity for improving his condition, by the exertion of which, he is carried on from one degree of advancement to another; and the similarity of his wants, as well as of the faculties by which those wants are supplied, has everywhere produced a remarkable uniformity in the several steps of his progression. (OR: 176)
However, just before this, Millar had commented upon the ‘amazing diversity’ of the laws and rules of conduct in different countries and at different times in the same country (175). And he swiftly proceeds to extend the diversity to encompass customs, taste, sentiments and the ‘general system of behaviour’ (176). The relationship between uniformity and diversity is the theme of this chapter; given that humans are the same across time and space then it is indeed prima facie ‘amazing’ that societies are so diverse.
At the very beginning of the opening chapter to the Ranks, Millar refers to ‘the most wonderful variety’ in the ‘rank and condition of women’ (183). We know from Chapter 3 that the ‘wonderful’ (or amazing) constituted a challenge - how is it to be explained, what causes are at work to account for it? For Millar an investigation of these causes will be ‘entertaining’ for it excites ‘our curiosity’ that there are such different rules of conduct (175). Alongside this simple desire to explain, Millar is a fully signed-up Baconian, this causal investigation will also be useful; without the knowledge attained ‘we cannot form a just notion of their [rules of conduct] utility’ (Ibid.).
In Chapter 2 we saw how much emphasis the Scots laid upon evidence. It was clear from the record, suitably sifted to weed out the prejudices of the recorders, that for every social institution from government to marriage to religion to ethical standards, reliable evidence could be found to indicate great differences in practice - whether it be, say, elected chiefs or polygamy or animism or probity. The acceptance of empiricism had one further consequence.