Moral necessity describes the will being (self) determined by an agent's reasons and motives. In the eighteenth-century debates about freedom and necessity (free will versus determinism), many thinkers distinguished a moral necessity from physical necessity and logical necessity. Extreme libertarians insisted on a will that was not determined by reasons or motives, fearing that this implies pre-determinism, which it does not. In two-stage models of free will, indeterminism in the generation of alternative possibilities for action breaks the causal chain of determinism. Not all the eighteenth century debates, by thinkers such as Samuel Clarke, William King, Anthony Collins, David Hume, and Thomas Reid, clearly distinguished logical, physical, and moral necessity. Logical necessity is a property of formal systems, predicate and propositional logic, and mathematics, for example. It is logically necessary that only one of two contradictory statements can be true, and the other false. This is the principle of bivalence or the law of the excluded middle. Physical necessity is the idea that everything that has ever happened and ever will happen is necessary, and can not be otherwise. Necessity is often opposed to chance and contingency. In a necessary world there is no chance. Everything that happens is necessitated. We usually describe this as physical determinism. Anthony Collins prefaced his 1717 essay, A Philosophical Inquiry Concerning Human Liberty, with this declaration,
When I affirm Necessity, I contend only for what is called Moral Necessity, meaning thereby, that man, who is an intelligent and sensible being, is determined by his reason and his senses; and I deny man to be subject to such necessity as is in clocks, watches, and such other beings, which for want of sensation and intelligence, are subject to an absolute, physical, or mechanical necessity. And here also I have the concurrence of almost all the greatest asserters of Liberty, who either expressly maintain moral necessity, or the thing signified by those words.Actions are determined by reasons, but not necessarily directly by the senses. The senses provide the conditions and context, the stimuli, in which an agent must generate alternative possibilities for action, the responses. If men were reactive machines, the responses would be determined directly by the stimuli. Samuel Clarke was the greatest defender of Newtonian physics and of natural religion based on Newtonian determinism for the physical universe. But Clarke thought the human mind, like the mind of God, was not physically determined. The immaterial mind has "libertarian free will." He claimed correctly that when the will was necessitated by motives, the "moral necessity" was not in fact, necessity at all (either physical or logical), except that once something is actually done, it is impossible that it not has been done.
As to the...necessity of the will's being determined by the last judgment of the understanding, this is only a necessity upon supposition, that is to say, a necessity that a man should will a thing, when it is supposed that he does will it; just as if one should affirm that everything which is, is therefore necessary to be, because when it is it cannot but be. It is exactly the same kind of argument as that by which the true church is proved to be infallible, because truth cannot err, and they who are in the right cannot possibly, while they are so, be in the wrong. Thus, whatever a man at any time freely wills or does, it is evident (even upon supposition of the most perfect liberty) that he cannot at that time but will or do it, because it is impossible anything should be willed and not willed, whether it be freely or necessarily, or that it should be done and not done at the same time. The necessity of the will's being determined by the last judgment of the understanding is, I say, only a necessity upon supposition, a necessity that a man should will a thing, when it is supposed that he does will it. For the last judgment of the understanding is nothing else but a man's final determining, after more or less consideration, either to choose or not to choose a thing; that is, it is the very same with the act of volition. Or else, if the act of volition he distinguished from the last judgment of the understanding, then the act of volition, or rather the beginning of action consequent upon the last judgment of the understanding, is not determined or caused by that last judgment as by the physical efficient, but only as the moral motive. For the true, proper, immediate, physical efficient cause of faction is the power of self-motion in men, which exerts itself freely in consequence of the last judgment of the understanding. But the last judgment of the understanding is not itself a physical efficient, but merely a moral motive upon which the physical efficient or motive power begins to act. The necessity, therefore, by which the power of acting follows the judgment of the understanding is only a moral necessity, that is, no necessity at all in the sense wherein the opposers of liberty understand necessity. For moral necessity is evidently consistent with the most perfect natural liberty.Logical philosophers argued that necessity implies actualism, that there is only one possible future, thus only one possible willed action. This is based on an ancient paradox developed by Diodorus Cronus, which claimed that only one of two contradictory statements about the future can be true and the other false. Only one answer to a question about a future event can be true. Either Yes or No. Actualism makes Clarke's "last judgment of the understanding" pre-determined by past events.
Aristotle solved Diodorus' paradox by saying that the truth of statements about the future was contingent on the actual future (although many philosophers, logicians, and mathematicians still have trouble with his conclusion)
"A sea battle must either take place tomorrow or not,And Clarke's morally necessitated will has the same contingent status. The priniciple of bivalence and the excluded middle apply to timeless (tenseless) logical propositions, but not to tensed potential empirical facts, before they become actual. The major founder of Stoicism, Chrysippus, took the edge off strict necessity. Like Democritus, Aristotle, and Epicurus before him, Chrysippus wanted to strengthen the argument for moral responsibility, in particular defending it from Aristotle's and Epicurus's indeterminate chance causes. Whereas the past is unchangeable, Chrysippus argued that some future events that are possible do not occur by necessity from past external factors alone, but might depend on us. We have a choice to assent or not to assent to an action. Later, Leibniz distinguished two forms of necessity, necessary necessity and contingent necessity. This basically distinguished logical necessity from physical (or empirical) necessity.
Chance is regarded as inconsistent with logical determinism and with any limits on causal, physical or mechanical determinism.
Despite abundant evidence to the contrary, many philosophers deny that chance exists. If a single event is determined by chance, then indeterminism would be true, they say, and undermine the very possibility of certain knowledge. Some go to the extreme of saying that chance would make the state of the world totally independent of any earlier states, which is nonsense, but it shows how anxious they are about chance.
Bertrand Russell said "The law of causation, according to which later events can theoretically be predicted by means of earlier events, has often been held to be a priori, a necessity of thought, a category without which science would not be possible." (Russell, External World p.179)
The core idea of indeterminism is closely related to the idea of causality. Indeterminism for some is simply an event without a cause. But we can have an adequate causality without strict determinism, which implies complete predictability of events and only one possible future.
An example of an event that is not strictly caused is one that depends on chance, like the flip of a coin. If the outcome is only probable, not certain, then the event can be said to have been caused by the coin flip, but the head or tails result was not predictable. So this causality, which recognizes prior events as causes, is undetermined and the result of chance alone.
We call this "soft" causality. Events are caused by prior (uncaused) events, but not determined by events earlier in the causal chain, which has been broken by the uncaused cause.
Necessity is critical for the question of free will. Strict necessity implies just one possible future. Chance means that the future is unpredictable. Chance allows alternative futures and the question becomes how the one actual present is realized from these potential alternatives.
The departure required from strict necessity is very slight compared to the miraculous ideas associated with the "causa sui" (self-caused cause) of the ancients.
Despite David Hume's critical attack on the necessity of causes, many philosophers embrace causality strongly. Some even connect it to the very possibility of logic and reason. And Hume himself strongly, if inconsistently, believed in necessity while denying causality. He said "'tis impossible to admit any medium betwixt chance and necessity."
Even in a world with chance, macroscopic objects are determined to an extraordinary degree. Newton's laws of motion are deterministic enough to send men to the moon and back. Our Cogito model of the Macro Mind is large enough to ignore quantum uncertainty for the purpose of the reasoning will. The neural system is robust enough to insure that mental decisions are reliably transmitted to our limbs.
We call this kind of determinism, limited as it is in extremely small structures, "adequate determinism." Determinism is adequate enough for us to predict eclipses for the next thousand years or more with extraordinary precision.
The presence of quantum uncertainty leads some philosophers to call the world indetermined. But indeterminism is misleading, with strong negative connotations, when most events are overwhelmingly "adequately determined."
There is no problem imagining that the three traditional mental faculties of reason - perception, conception, and comprehension - are for all practical purposes carried on deterministically in a physical brain where quantum events do not interfere with normal operations, unless the agent deliberately seeks to be original and creative.
There is also no problem imagining a role for chance in the brain in the form of quantum level noise. Noise can introduce random errors into stored memories. Noise could create random associations of ideas during memory recall which are the source of novel ideas.
[Necessity must be limited to its proper use in logic, and disambiguated from its close relatives causality, determinism, certainty, and predictability.]