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 rules.