t 'Man is mortal' predicates causation: the human
constitution issues in death.

The explanation of this inconsistency may perhaps be found in the
history of Mill's work. Books I. and II. were written in 1831; but being
unable at that time to explain Induction, he did not write Book III.
until 1837-8. Then, no doubt, he revised the earlier Books, but not
enough to bring his theory of the syllogism into complete agreement with
the theory of Induction; so that the axiom of co-existence was allowed
to stand.

Mill also introduced the doctrine of Natural Kinds as a ground of
Induction supplementary, at least provisionally, to causation; and to
reasoning about Kinds, or Substance and Attribute, his axiom of
co-existence is really adapted. Kinds are groups of things that agree
amongst themselves and differ from all others in a multitude of
qualities: these qualities co-exist, or co-inhere, with a high degree of
constancy; so that where some are found others may be inferred. Their
co-inherence is not to be considered an ultimate fact; for, "since
everything which occurs is determined by laws of causation and
collocations of the original causes, it follows that the co-existences
observable amongst effects cannot themselves be the subject of any
similar set of laws distinct from laws of causation" (B. III. c. 5, Sec.
9). According to the theory of evolution (worked out since Mill wrote),
Kinds--that is, species of plants, animals and minerals--with their
qualities are all due to causation. Still, as we can rarely, or never,
trace the causes with any fullness or precision, a great deal of our
reasoning, as, e.g., about men and camels, does in fact trust to the
relative permanence of natural Kinds as defined by co-inhering
attributes.

To see this more clearly, we should consider that causation and natural
Kinds are not at present separable; propositions about causation in
concrete phenomena (as distinct from abstract 'forces') always involve
the assumption of Kinds. For example--'Water rusts iron,' or the oxygen
of water combines with iron immersed in it to form rust: this statement
of causation assumes that water, oxygen, iron, and iron-rust are known
Kinds. On the other hand, the constitution of every concrete thing, and
manifestly of every organised body, is always undergoing change, that
is, causation, upon which fact its properties depend.

How, then, can we frame principles of mediate reasoning, about such
things? So f