_morally_ that, if he would
continue to live, he should eat and sleep, food and rest being,
according to the established constitution of Nature, a _necessary
condition_ or indispensable means for the support of life. There is in
like manner a "moral necessity" that we should be virtuous and obedient,
if we would be truly happy, virtue and obedience being, according to the
established constitution of Nature, an indispensable means of true and
permanent happiness. This is "moral necessity" which has reference
solely to the connection between _means_ and _ends_, but that
connection, being ordained, is immutable and invariable.

"Physical necessity," again, exists wherever there is either a causal
connection between antecedents and consequents in the material world, or
even a coactive and compulsory constraint in the moral world. It is
physically necessary that fire should burn substances that are
combustible, that water and other fluids should flow down a declivity,
and rise again but only to a certain level; and there is the like kind
of necessity, wherever a moral agent is forced to act under irresistible
compulsion,--as when the assassin seizes hold of another's arm, and
thrusting a deadly weapon into his hand, directs it, by his own
overmastering will, to the brain or heart of his victim. In this latter
case, the unwilling instrument of his revenge or malice is not held to
be the guilty party, but the more powerful agent by whom that instrument
was employed. This is "physical necessity," which relates solely to the
connection between cause and effect in the material world, and, in the
moral, to the compulsory action of one agent on another.

"Metaphysical necessity," again, can be predicated of God only, and
denotes the peculiar property or prerogative of His being, as existing
necessarily, immutably, and eternally, or, to use a scholastic phrase,
the necessary connection in His case between _essence_ and _existence_.

Omitting the _last_, which does not fall properly within the limits of
our present inquiry, we may say with regard to _the three first_, that
each of them may exist, and that each of them does really operate, in
the present constitution of Nature. We are subject, unquestionably, to
certain "laws of thought," which we can neither repeal nor resist, and
which impose upon us a logical necessity to conceive, to reason, and to
infer, not according to our own whim or caprice, but according to
established rul