n facts of immediate experience, which, in virtue of endless
repetition, we regard as necessary.

The probability of a common cause, and the possibility of formulating
it, being granted, it will be well, first, to ask what must be the
general characteristics of such cause, and in what direction we ought to
look for it. We can with certainty predict that it has a high degree of
abstractness; seeing that it is common to such infinitely-varied
phenomena. We need not expect to see in it an obvious solution of this
or that form of progress; because it is equally concerned with forms of
progress bearing little apparent resemblance to them: its association
with multiform orders of facts, involves its dissociation from any
particular order of facts. Being that which determines progress of every
kind--astronomic, geologic, organic, ethnologic, social, economic,
artistic, &c.--it must be involved with some fundamental trait displayed
in common by these; and must be expressible in terms of this fundamental
trait. The only obvious respect in which all kinds of progress are
alike, is, that they are modes of _change_; and hence, in some
characteristic of changes in general, the desired solution will probably
be found. We may suspect _a priori_ that in some universal law of change
lies the explanation of this universal transformation of the
homogeneous into the heterogeneous.

Thus much premised, we pass at once to the statement of the law, which
is this:--_Every active force produces more than one change--every cause
produces more than one effect._

To make this proposition comprehensible, a few examples must be given.
When one body strikes another, that which we usually regard as the
effect, is a change of position or motion in one or both bodies. But a
moment's thought shows us that this is a very incomplete view of the
matter. Besides the visible mechanical result, sound is produced; or, to
speak accurately, a vibration in one or both bodies, which is
communicated to the surrounding air; and under some circumstances we
call this the effect. Moreover, the air has not only been made to
undulate, but has had currents caused in it by the transit of the
bodies. Further, there is a disarrangement of the particles of the two
bodies in the neighbourhood of their point of collision; amounting, in
some cases, to a visible condensation. Yet more, this condensation is
accompanied by the disengagement of heat. In some cases a spark--that
is