compare its two most unlike kinds
and see in what they agree.

Choosing _assimilation_, then, for our example of bodily life, and
_reasoning_ for our example of the life known as intelligence, it is
first to be observed that they are both processes of change. Without
change food cannot be taken into the blood nor transformed into tissue:
neither can conclusions be obtained from premises. This conspicuous
manifestation of change forms the substratum of our idea of life in
general. Comparison shows this change to differ from non-vital changes
in being made up of _successive_ changes. The food must undergo
mastication, digestion, etc., while an argument necessitates a long
chain of states of consciousness, each implying a change of the
preceding state. Vital change is further made up of many _simultaneous_
changes. Assimilation and argument both include many actions going on
together. Vital changes, both visceral and cerebral, also differ from
other changes in their _heterogeneity_; neither the simultaneous nor the
serial acts of digestion or of ratiocination are at all alike. They are
again distinguished by the combination subsisting among their
constituent changes. The acts that make up digestion are mutually
dependent; as are those which compose a train of reasoning. Once more,
they differ in being characterised by definiteness. Assimilation,
respiration, and circulation, are definitely interdependent. These
characterisations not only mark off the vital from the non-vital, but
also creatures of high vitality from those of low vitality. Hence our
formula reads thus:--_Life is the definite combination of heterogeneous
changes, both simultaneous and successive._ Not _a_ definite
combination, allowing that there may be others, but _the_ definite
combination. This, however, omits its most distinctive peculiarity.

_Correspondence Between Life and Its Circumstances_

We habitually distinguish between a live object and a dead one by
observing whether a change in the surrounding conditions is or is not
followed by some perceptible and appropriate change in the object.
Adding this all-important characteristic, our conception of life
becomes--the definite combination of heterogeneous changes, both
simultaneous and successive, _in correspondence with external
coexistences and sequences_. Some illustrations may serve to show the
significance of this addition.

Every act of locomotion implies the expenditure of certain intern