rious errors is as good
as that of anyone else, who might have been persuaded to undertake the
somewhat perilous enterprise in which I find myself engaged.

There is yet another prefatory remark which it seems desirable I
should make. It is that I think it proper to confine myself to the
work done, without saying anything about the doers of it. Meddling
with questions of merit and priority is a thorny business at the best
of times, and unless in case of necessity, altogether undesirable when
one is dealing with contemporaries. No such necessity lies upon me,
and I shall, therefore, mention no names of living men, lest,
perchance, I should incur the reproof which the Israelites, who
struggled with one another in the field, addressed to Moses--'Who made
thee a prince and a judge over us.'

[Sidenote: The aim of physical science]

Physical science is one and indivisible. Although, for practical
purposes, it is convenient to mark it out into the primary regions of
Physics, Chemistry, and Biology, and to subdivide these into
subordinate provinces, yet the method of investigation and the
ultimate object of the physical inquirer are everywhere the same.

[Sidenote: the discovery of the rational order of the universe]

The object is the discovery of the rational order which pervades the
universe, the method consists of observation and experiment (which is
observation under artificial conditions) for the determination of the
facts of nature, of inductive and deductive reasoning for the
discovery of their mutual relations and connection. The various
branches of physical science differ in the extent to which at any
given moment of their history, observation on the one hand, or
ratiocination on the other, is their more obvious feature, but in no
other way, and nothing can be more incorrect than the assumption one
sometimes meets with, that physics has one method, chemistry another,
and biology a third.

[Sidenote: It is based on postulates]

All physical science starts from certain postulates. One of them is
the objective existence of a material world. It is assumed that the
phenomena which are comprehended under this name have a 'substratum'
of extended, impenetrable, mobile substance, which exhibits the
quality known as inertia, and is termed matter.[E] Another postulate
is the universality of the law of causation; that nothing happens
without a cause (that is, a necessary precedent condition), and that
the state of the