tever is, whether in
geology, astronomy, biology, or sociology, is the result of numberless,
inevitable, related changes. Only the law of change is changeless. The
present is a phase only of a great transition process from what was,
through what is, to what will be.

The Marx-Engels theory is an exploration of the laws governing this
process of evolution in the domain of human relations: an attempt to
provide a key to the hitherto mysterious succession of changes in the
political, juridical, and social relations and institutions of mankind.
Whence, for instance, arose the institution of chattel slavery, so
repugnant to our modern ideas of right and wrong, and how shall we
explain its defense and justification in the name of religion and
morality? How account for the fact that what Yesterday regarded as
righteous, To-day condemns as wrong; that what at one period of the
world's history is regarded as perfectly natural and right--the practice
of polygamy, for example--becomes abhorrent at another period; or that
what is regarded with horror and disgust in one part of the world is
sanctioned by the ethical codes, and freely practiced elsewhere? Ferri
gives two examples of this kind: the cannibalism of Central African
tribes, and the killing of parents, as a religious duty, in Sumatra.[61]
To reply "custom" is to beg the whole question, for customs do not exist
without reason, however difficult it may be to discern the reason for
any particular custom. To reply that these things are mysteries, as the
old theologians did when the doctrine of the Trinity was questioned, is
to leave the question unanswered and to challenge doubt and
investigation. The human mind abhors a mystery as nature abhors a
vacuum. Despite Spencer, the human mind has never admitted the existence
of the _Unknowable_. To explore the _Unknown_ is man's universal
impulse; and with each fresh discovery the _Unknown_ is narrowed by the
expansion of the _Known_.

The theory that ideas determine progress, that, in the words of
Professor Richard T. Ely, "all that is significant in human history may
be traced back to ideas,"[62] is only true in the sense that a half
truth is true. It is true, nothing but the truth, but it is less than
the whole truth. Truly all that is significant in human history may be
traced back to ideas, but in like manner the ideas themselves can be
traced back to material sources. For ideas have histories, too, and the
causation of an id