INTRODUCTORY 13

of modifying influences, thus providing a broader basis on
which to push the @ priori argument further. Fresh
guidance, for correction of the hypotheses of the argument so
as to bring them into accordance with actuality, may then be
sought in historical enquiry. Thus the two methods may,
and must, proceed hand in hand. They have done so to a
large extent. The method followed in the present volume
is mainly the a priori method, but the use of this method is
not meant to imply the neglect of the other. The broader
generalisations need constant qualification to adapt them
for application to special groups or conditions, and the
appropriate qualifications can only be discovered, in general,
by a study of various sets of facts. The frequent appeal to
statistics which is made by theoretic writers on economic
subjects, is an example of this reliance on historical methods
of enquiry, for the data of their arguments, or to provide
illustrations of the operation of the influences whose effects
they are discussing.
The data of economic arguments are comprised in three
principal groups of facts. First, the facts of physical nature,
such as those relating to the way in which the return to
effort expended in the cultivation of land varies with the
amount of effort devoted to any particular area. Secondly,
the facts of human nature, such as the mode and extent of
occurrence of self-regarding motives in economic action, and
of regard for members of the same family, tribe, nation,
trade, or profession; the degree of enterprise, intelligence,
foresight, trustworthiness, caution, etc., displayed by any
economic group which is the subject of study. Thirdly,
such facts of social organisation as the nature of the rights of
property, laws relating to inheritance, bankruptcy, etc., which
prevail in the society which is the subject of study.
These different sets of facts are not related to economic
argument in quite the same way. The first group must be
accepted as practically unalterable, but the others are undergoing
 change in any particular community, though many of
them change but slowly, and some change the mode of their
manifestation rather than their real nature. In different communities,
 the differences in the characters of the people, and