les, which may be distinguished in it as follows:

(1) The Principles of Contradiction and Excluded Middle (ch. vi. Sec. 3)
declare that in a given relation to a given phenomenon any two or more
other phenomena are incompatible (_B is not A and a_); whilst the given
phenomenon either stands related to another phenomenon or not (_B is
either A or a_). It is not only a matter of Logic but of fact that, if a
leaf is green, it is not under the same conditions red or blue, and that
if it is not green it is some other colour.

(2) Certain Axioms of Mediate Evidence: as, in Mathematics, 'that
magnitudes equal to the same magnitude are equal to one another'; and,
in Logic, the _Dictum_ or its material equivalent.

(3) That all Times and all Spaces are commensurable; although in certain
relations of space (as [pi]) the unit of measurement must be infinitely
small.--If Time really trotted with one man and galloped with another,
as it seems to; if space really swelled in places, as De Quincey dreamed
that it did; life could not be regulated, experience could not be
compared and science would be impossible. The Mathematical Axioms would
then never be applicable to space or time, or to the objects or
processes that fill them.

(4) The Persistence of Matter and Energy: the physical principle that,
in all changes of the universe, the quantities of Matter and Energy
(actual and potential, so-called) remain the same.--For example, as to
matter, although dew is found on the grass at morning without any
apparent cause, and although a candle seems to burn away to a scrap of
blackened wick, yet every one knows that the dew has been condensed from
vapour in the air, and that the candle has only turned into gas and
smoke. As to energy, although a stone thrown up to the housetop and
resting there has lost actual energy, it has gained such a position that
the slightest touch may bring it to the earth again in the same time as
it took to travel upwards; so on the house-top it is said to have
potential energy. When a boiler works an engine, every time the piston
is thrust forward (mechanical energy), an equivalent in heat (molecular
energy) is lost. But for the elucidation of these principles, readers
must refer to treatises of Chemistry and Physics.

(5) Causation, a special form of the foregoing principles of the
persistence of matter and energy, we shall discuss in the next chapter.
It is not to be conceived of as anything occult or noumen