This, however, is by no means all that is comprehended in the education
that prepares for direct self-preservation. Besides guarding the body
against mechanical damage or destruction, it has to be guarded against
injury from other causes--against the disease and death that follow
breaches of physiologic law. For complete living it is necessary, not
only that sudden annihilations of life shall be warded off; but also
that there shall be escaped the incapacities and the slow annihilation
which unwise habits entail. As, without health and energy, the
industrial, the parental, the social, and all other activities become
more or less impossible; it is clear that this secondary kind of direct
self-preservation is only less important than the primary kind; and
that knowledge tending to secure it should rank very high.

It is true that here, too, guidance is in some measure ready supplied.
By our various physical sensations and desires, Nature has insured a
tolerable conformity to the chief requirements. Fortunately for us, want
of food, great heat, extreme cold, produce promptings too peremptory to
be disregarded. And would men habitually obey these and all like
promptings when less strong, comparatively few evils would arise. If
fatigue of body or brain were in every case followed by desistance; if
the oppression produced by a close atmosphere always led to ventilation;
if there were no eating without hunger, or drinking without thirst; then
would the system be but seldom out of working order. But so profound an
ignorance is there of the laws of life, that men do not even know that
their sensations are their natural guides, and (when not rendered morbid
by long--continued disobedience) their trustworthy guides. So that
though, to speak teleologically, Nature has provided efficient
safeguards to health, lack of knowledge makes them in a great measure
useless.