Will, generally, is that faculty of the mind which selects, at the moment of decision, the strongest desire from among the various desires present. Will does not refer to any particular desire, but rather to the mechanism for choosing from among one's desires. Within philosophy the will is important as one of the distinct parts of the mind - along with reason and understanding. It is considered central to the field of ethics because of its role in enabling deliberate action.
One of the recurring questions discussed in the Western philosophical tradition is that of free will - and the related, but more general notion of fate - which asks how the will can be truly free if a person's actions have either natural or divine causes which determine them. In turn, this is directly connected to discussions on the nature of freedom itself and to the problem of evil.
The classical treatment of the ethical importance of will is to be found in the Nicomachean Ethics of Aristotle, in Books III (chapters 1-5), and Book VII (chapters 1-10). These discussions have been a major influence in the development of ethical and legal thinking in western civilization.
In Book III Aristotle divided actions into three categories instead of two:
It is concerning this third class of actions that there is doubt about whether they should be praised or blamed or condoned in different cases.
Virtue and vice according to Aristotle are "up to us". This means that although no one is willingly unhappy, vice by definition always involves actions which were decided upon willingly. Vice comes from bad habits and aiming at the wrong things, not deliberately aiming to be unhappy. The vices then, are voluntary just as the virtues are. He states that people would have to be unconscious not to realize the importance of allowing themselves to live badly, and he dismisses any idea that different people have different innate visions of what is good.
In Book VII, Aristotle discusses self-mastery, or the difference between what people decide to do, and what they actually do. For Aristotle, akrasia, "unrestraint", is distinct from animal-like behavior because it is specific to humans and involves conscious rational thinking about what to do, even though the conclusions of this thinking are not put into practice. When someone behaves in a purely animal-like way, then for better or worse they are not acting based upon any conscious choice.
Aristotle also addresses a few questions raised earlier, on the basis of what he has explained:
Inspired by Islamic philosophers Avicenna and Averroes, Aristotelian philosophy became part of a standard approach to all legal and ethical discussion in Europe by the time of Thomas Aquinas. His philosophy can be seen as a synthesis of Aristotle and early Christian doctrine as formulated by Boethius and Augustine of Hippo, although sources such as Maimonides and Plato and the aforementioned Muslim scholars are also cited.
The use of English in philosophical publications began in the early modern period, and therefore the English word "will" became a term used in philosophical discussion. During this same period, Scholasticism, which had largely been a Latin language movement, was heavily criticized. Both Francis Bacon and René Descartes described the human intellect or understanding as something which needed to be considered limited, and needing the help of a methodical and skeptical approach to learning about nature. Bacon emphasized the importance of analyzing experience in an organized way, for example experimentation, while Descartes, seeing the success of Galileo in using mathematics in physics, emphasized the role of methodical reasoning as in mathematics and geometry. Descartes specifically said that error comes about because the will is not limited to judging things which the understanding is limited to, and described the possibility of such judging or choosing things ignorantly, without understanding them, as free will. Dutch theologian Jacobus Arminius, considered the freedom of human will is to work toward individual salvation and constrictions occur due to the work of passion that a person holds. Augustine calls will as "the mother and guardian of all virtues". 
Under the influence of Bacon and Descartes, Thomas Hobbes made one of the first attempts to systematically analyze ethical and political matters in a modern way. He defined will in his Leviathan Chapter VI, in words which explicitly criticize the medieval scholastic definitions:
In deliberation, the last appetite, or aversion, immediately adhering to the action, or to the omission thereof, is that we call the will; the act, not the faculty, of willing. And beasts that have deliberation, must necessarily also have will. The definition of the will, given commonly by the Schools, that it is a rational appetite, is not good. For if it were, then could there be no voluntary act against reason. For a voluntary act is that, which proceedeth from the will, and no other. But if instead of a rational appetite, we shall say an appetite resulting from a precedent deliberation, then the definition is the same that I have given here. Will therefore is the last appetite in deliberating. And though we say in common discourse, a man had a will once to do a thing, that nevertheless he forbore to do; yet that is properly but an inclination, which makes no action voluntary; because the action depends not of it, but of the last inclination, or appetite. For if the intervenient appetites, make any action voluntary; then by the same reason all intervenient aversions, should make the same action involuntary; and so one and the same action, should be both voluntary and involuntary.
By this it is manifest, that not only actions that have their beginning from covetousness, ambition, lust, or other appetites to the thing propounded; but also those that have their beginning from aversion, or fear of those consequences that follow the omission, are voluntary actions.
Concerning "free will", most early modern philosophers, including Hobbes, Spinoza, Locke and Hume believed that the term was frequently used in a wrong or illogical sense, and that the philosophical problems concerning any difference between "will" and "free will" are due to verbal confusion (because all will is free):
a FREEMAN, is he, that in those things, which by his strength and wit he is able to do, is not hindered to do what he has a will to. But when the words free, and liberty, are applied to any thing but bodies, they are abused; for that which is not subject to motion, is not subject to impediment: and therefore, when it is said, for example, the way is free, no liberty of the way is signified, but of those that walk in it without stop. And when we say a gift is free, there is not meant any liberty of the gift, but of the giver, that was not bound by any law or covenant to give it. So when we speak freely, it is not the liberty of voice, or pronunciation, but of the man, whom no law hath obliged to speak otherwise than he did. Lastly, from the use of the word free-will, no liberty can be inferred of the will, desire, or inclination, but the liberty of the man; which consisteth in this, that he finds no stop, in doing what he has the will, desire, or inclination to do.."
Spinoza argues that seemingly "free" actions aren't actually free, or that the entire concept is a chimera because "internal" beliefs are necessarily caused by earlier external events. The appearance of the internal is a mistake rooted in ignorance of causes, not in an actual volition, and therefore the will is always determined. Spinoza also rejects teleology, and suggests that the causal nature along with an originary orientation of the universe is everything we encounter.
Some generations later, David Hume made a very similar point to Hobbes in other words:
But to proceed in this reconciling project with regard to the question of liberty and necessity; the most contentious question of metaphysics, the most contentious science; it will not require many words to prove, that all mankind have ever agreed in the doctrine of liberty as well as in that of necessity, and that the whole dispute, in this respect also, has been hitherto merely verbal. For what is meant by liberty, when applied to voluntary actions? We cannot surely mean that actions have so little connexion with motives, inclinations, and circumstances, that one does not follow with a certain degree of uniformity from the other, and that one affords no inference by which we can conclude the existence of the other. For these are plain and acknowledged matters of fact. By liberty, then, we can only mean a power of acting or not acting, according to the determinations of the will; that is, if we choose to remain at rest, we may; if we choose to move, we also may. Now this hypothetical liberty is universally allowed to belong to every one who is not a prisoner and in chains. Here, then, is no subject of dispute.
Jean-Jacques Rousseau added a new type of will to those discussed by philosophers, which he called the "General will" (volonté générale). This concept developed from Rousseau's considerations on the social contract theory of Hobbes, and describes the shared will of a whole citizenry, whose agreement is understood to exist in discussions about the legitimacy of governments and laws.
Kant's Transcendental Idealism claimed that "all objects are mere appearances [phenomena]." He asserted that "nothing whatsoever can ever be said about the thing in itself that may be the basis of these appearances." Kant's critics responded by saying that Kant had no right, therefore, to assume the existence of a thing in itself.
Schopenhauer disagreed with Kant's critics and stated that it is absurd to assume that phenomena have no basis. Schopenhauer proposed that we cannot know the thing in itself as though it is a cause of phenomena. Instead, he said that we can know it by knowing our own body, which is the only thing that we can know at the same time as both a phenomenon and a thing in itself.
When we become conscious of ourself, we realize that our essential qualities are endless urging, craving, striving, wanting, and desiring. These are characteristics of that which we call our will. Schopenhauer affirmed that we can legitimately think that all other phenomena are also essentially and basically will. According to him, will "is the innermost essence, the kernel, of every particular thing and also of the whole. It appears in every blindly acting force of nature, and also in the deliberate conduct of man…." Schopenhauer said that his predecessors mistakenly thought that the will depends on knowledge. According to him, though, the will is primary and uses knowledge in order to find an object that will satisfy its craving. That which, in us, we call will is Kant's "thing in itself", according to Schopenhauer.
Arthur Schopenhauer put the puzzle of free will and moral responsibility in these terms:
Everyone believes himself a priori to be perfectly free, even in his individual actions, and thinks that at every moment he can commence another manner of life ... But a posteriori, through experience, he finds to his astonishment that he is not free, but subjected to necessity, that in spite of all his resolutions and reflections he does not change his conduct, and that from the beginning of his life to the end of it, he must carry out the very character which he himself condemns...
In his On the Freedom of the Will, Schopenhauer stated, "You can do what you will, but in any given moment of your life you can will only one definite thing and absolutely nothing other than that one thing."
Friedrich Wilhelm Nietzsche was influenced by Schopenhauer when younger, but later felt him to be wrong. However, he maintained a modified focus upon will, making the term "will to power" famous as an explanation of human aims and actions.
Psychologists also deal with issues of will and "willpower" the ability to affect will in behavior; some people are highly intrinsically motivated and do whatever seems best to them, while others are "weak-willed" and easily suggestible (extrinsically motivated) by society or outward inducement. Apparent failures of the will and volition have also been reported associated with a number of mental and neurological disorders. They also study the phenomenon of Akrasia, wherein people seemingly act against their best interests and know that they are doing so (for instance, restarting cigarette smoking after having intellectually decided to quit). Advocates of Sigmund Freud's psychology stress the importance of the influence of the unconscious mind upon the apparent conscious exercise of will. Abraham Low, a critic of psychoanalysis, stressed the importance of will, the ability to control thoughts and impulses, as fundamental for achieving mental health.