Naturalism in philosophy, as it is in science, is the search for explanations that involve only Nature, ones that in particular do not involve supernatural ideas.

Metaphysical or ontological naturalism is the idea that there is nothing in the world but Nature. This lead to difficulties as to the existential status of ideas, abstract concepts like justice, and entities like numbers or a geometric circle.

Methodological naturalism accepts as explanations only arguments based on natural phenomena. If and when abstract ideas are properly understood, it will be because they have natural explanations.

Ethical naturalism moves the question of values and their origin outward from early humanist views, first to biological explanations (the evolution of ethics in higher organisms), but ultimately to the universe as a whole. Moral skeptics from Thomas Hobbes to Friedrich Nietzsche see ethics as invented for reasons of self-interest in a social contract.

Natural religion is an attempt to explain religious beliefs about the creation of the universe in wholly natural terms. Though some see this as a conflicted and futile attempt to naturalize supernaturalism, the philosophy of religion began in earnest with David Hume's Dialogues Concerning Natural Religion and The Natural History of Religion.

Naturalism has a long history in the free will debates, beginning with Hume's arguments in the Treatise on Human Nature and the Enquiries that humans have "natural beliefs" that are prior to experience and shape our perceptions.

Anticipating Immanuel Kant's synthetic a priori, Hume argued that a skeptical view of empiricism prevented us from knowing basic things like causality and the external world, but that a "natural belief" in causality and the external world could not be negated by any skeptical arguments.

Hume hoped to build a science of Human Nature modeled on Isaac Newton's Principia, which had become the canonical model for all science. But his reintroduction of mitigated academic skepticism made any science at all problematic.

Epistemological theories that all knowledge was based on reasoning about sense data perceived by a mind that began as a blank slate run into the criticism that we can only know those sense data, and not the "things themselves" in the external world that are producing the perceptions.

For the Scottish School of philosophy, which strongly influenced Hume, transcendental beliefs could trump reason. They were prior to reason. Hume argued that we could not reason without beliefs, desires, and passions. Indeed, he argued that an act of will was driven by beliefs and desires, never by reason, which was merely an instrument to evaluate various means to our ends. This was similar to the position of Scholastic philosophers like Thomas Aquinas.

Natural beliefs that Hume felt could not be denied by reasoned arguments, such as the principle of uniformity and the existence of the external world, were incorporated by Kant into his transcendental theory that the mind imposed categories of understanding on the world. Kant's "synthetic a priori" claimed to establish certain truths about the world that could be known without empirical, a posteriori, studies of the world.

Among Kant's attempts at synthetic a priori truths were Euclidean geometry and determinism.

Kant's arguments that we must limit reason to make room for beliefs seem similar to Hume's view that some beliefs necessarily precede any reason. Both the Humean and Kantian projects are best seen as trying to establish morality in an age of empirical and deterministic science.

Today these beliefs are regarded as assumptions or axioms that are tested by their explanatory power in empirical science. But science and pure reason seem unable to deal with questions of free will and moral responsibility, which for Hume and Kant (and later Wittgenstein) were all-important.

Hume and Hobbes were the two leading compatibilists of their times, believing that free will was compatible with strict determinism. Both denied the reality of absolute chance. For them, chance was the result of human ignorance. Chance is an epistemic question, not an ontological problem.

But in contrast to Hobbes' moral skepticism and the supremacy of self-interest, Hume hoped to establish the foundations of a morality based on natural moral sentiment in An Enquiry concerning the Principles of Morals, Part II

Self-love is a principle in human nature of such extensive energy, and the interest of each individual is, in general, so closely connected with that of the community, that those philosophers were excusable, who fancied, that all our concern for the public might be resolved into a concern for our own happiness and preservation. They saw every moment, instances of approbation or blame, satisfaction or displeasure towards characters and actions; they denominated the objects of these sentiments, virtues, or vices; they observed, that the former had a tendency to encrease the happiness, and the latter the misery of mankind; they asked, whether it were possible that we could have any general concern for society, or any disinterested resentment of the welfare or injury of others; they found it simpler to consider all these sentiments as modifications of self-love; and they discovered a pretence, at least, for this unity of principle, in that close union of interest, which is so observable between the public and each individual.

But notwithstanding this frequent confusion of interests, it is easy to attain what natural philosophers, after Lord Bacon, have affected to call the experimentum crucis, or that experiment, which points out the right way in any doubt or ambiguity. We have found instances, in which private interest was separate from public; in which it was even contrary; And yet we observed the moral sentiment to continue, notwithstanding this disjunction of interests. And wherever these distinct interests sensibly concurred, we always found a sensible encrease of the sentiment, and a more warm affection to virtue, and detestation of vice, or what we properly call, gratitude and revenge. Compelled by these instances, we must renounce the theory, which accounts for every moral sentiment by the principle of self-love. We must adopt a more public affection, and allow, that the interests of society are not, even on their own account, entirely indifferent to us. Usefulness is only a tendency to a certain end; and it is a contradiction in terms, that any thing pleases as means to an end, where the end itself no wise affects us. If usefulness, therefore, be a source of moral sentiment, and if this usefulness be not always considered with a reference to self; it follows, that every thing, which contributes to the happiness of society recommends itself directly to our approbation and good-will. Here is a principle, which accounts, in great part, for the origin of morality: And what need we seek for abstruse and remote systems, when there occurs one so obvious and natural?

Hume gives the argument for moral sentiment as superior to reason or judgment in Appendix I, Concerning Moral Sentiment, though reason helps with calculations of utility.

If the foregoing hypothesis be received, it will now be easy for us to determine the question first started, concerning the general principles of morals; and though we postponed the decision of that question, lest it should then involve us in intricate speculations, which are unfit for moral discourses, we may resume it at present, and examine how far either reason or sentiment enters into all decisions of praise or censure.

One principal foundation of moral praise being supposed to lie in the usefulness of any quality or action; it is evident, that reason must enter for a considerable share in all decisions of this kind; since nothing but that faculty can instruct us in the tendency of qualities and actions, and point out their beneficial consequences to society and to their possessors...And a very accurate reason or judgment is often requisite, to give the true determination, amidst such intricate doubts arising from obscure or opposite utilities.

But though reason, when fully assisted and improved, be sufficient to instruct us in the pernicious or useful tendency of qualities and actions; it is not alone sufficient to produce any moral blame or approbation. Utility is only a tendency to a certain end; and were the end totally indifferent to us, we should feel the same indifference towards the means. It is requisite a sentiment should here display itself, in order to give a preference to the useful above the pernicious tendencies. This sentiment can be no other than a feeling for the happiness of mankind, and a resentment of their misery; since these are the different ends which virtue and vice have a tendency to promote. Here, therefore, reason instructs us in the several tendencies of actions, and humanity makes a distinction in favour of those which are useful and beneficial.

In the famous passage where Hume shows that "Ought" cannot be derived from "Is," he again makes the case for natural passions, motives, volitions, thoughts, and feelings as the source for sentiments of morality. There is no matter of fact discernible by reason alone.(Treatise, Book III, Sect I, p.468)

Nor does this reasoning only prove, that morality consists not in any relations, that are the objects of science; but if examin'd, will prove with equal certainty, that it consists not in any matter of fact, which can be discover'd by the understanding. This is the second part of our argument; and if it can be made evident, we may conclude, that morality is not an object of reason. But can there be any difficulty in proving, that vice and virtue are not matters of fact, whose existence we can infer by reason? Take any action allow'd to be vicious: Wilful murder, for instance. Examine it in all lights, and see if you can find that matter of fact, or real existence, which you call vice. In which-ever way you take it, you find only certain passions, motives, volitions and thoughts. There is no other matter of fact in the case. The vice entirely escapes you, as long as you consider the object. You never can find it, till you turn your reflection into your own breast, and find a sentiment of disapprobation, which arises in you, towards this action. Here is a matter of fact; but `tis the object of feeling, not of reason. It lies in yourself, not in the object. So that when you pronounce any action or character to be vicious, you mean nothing, but that from the constitution of your nature you have a feeling or sentiment of blame from the contemplation of it. Vice and virtue, therefore, may be compar'd to sounds, colours, heat and cold, which, according to modern philosophy, are not qualities in objects, but perceptions in the mind: And this discovery in morals, like that other in physics, is to be regarded as a considerable advancement of the speculative sciences; tho', like that too, it has little or no influence on practice. Nothing can be more real, or concern us more, than our own sentiments of pleasure and uneasiness; and if these be favourable to virtue, and unfavourable to vice, no more can be requisite to the regulation of our conduct and behaviour.

Hume the skeptic doubts ought can be derived from is. Hume the naturalist has no such problem

I cannot forbear adding to these reasonings an observation, which may, perhaps, be found of some importance. In every system of morality, which I have hitherto met with, I have always remark'd, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surpriz'd to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, `tis necessary that it shou'd be observ'd and explain'd; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention wou'd subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceiv'd by reason.

Perhaps the most important recent discussion of naturalism and free will is Peter Strawson's 1962 essay Freedom and Resentment, which changed the subject from the truth of determinism or free will to the Humean claim that moral attitudes exist quite independently of the reasoned "truth" of determinism or the free will thesis.

This is of course also Hume's position, since no reasoned argument can cause us to abandon our natural beliefs that lead to sympathy with others and feelings of gratitude and resentment.

Surprisingly, this famous Strawson essay has only a single reference to Hume, a footnote on Hume's denial of any "rational" justification of induction. So, says Strawson, there is no rational denial of moral responsibility, based on what he calls the reactive attitudes. This argument leads directly to John Martin Fischer's semi-compatibilism.

Strawson arrays "pessimists" - genuine moral skeptics - against "optimists" - apparently compatibilists - and hopes to reconcile them:

Some philosophers say they do not know what the thesis of determinism is. Others say, or imply, that they do know what it is. Of these, some — the pessimists perhaps — hold that if the thesis is true, then the concepts of moral obligation and responsibility really have no application, and the practices of punishing and blaming, of expressing moral condemnation and approval, are really unjustified. Others—the optimists perhaps—hold that these concepts and practices in no way lose their raison d’être if the thesis of determinism is true. Some hold even that the justification of these concepts and practices requires the truth of the thesis. There is another opinion which is less frequently voiced: the opinion, it might be said, of the genuine moral sceptic.

Note the standard two-part argument against free will

This is that the notions of moral guilt, of blame, of moral responsibility are inherently confused and that we can see this to be so if we consider the consequences either of the truth of determinism or of its falsity. The holders of this opinion agree with the pessimists that these notions lack application if determinism is true, and add simply that they also lack it if determinism is false. If I am asked which of these parties I belong to, I must say it is the first of all, the party of those who do not know what the thesis of determinism is. But this does not stop me from having some sympathy with the others, and a wish to reconcile them.

In his 1985 book Skepticism and Naturalism, Strawson describes two naturalisms, a "reductive naturalism" (which he also calls strict or hard) and another naturalism, perhaps his own view (which he calls liberal, catholic, or soft).

He connects reductive naturalism to skepticism and scientism, which he feels denies some evident truths and realities (such as the existence of the world), but thinks the liberal naturalist might be accused of fostering illusions or propagating myths. He then applies these two approaches to his reactive moral attitudes (Skepticism and Naturalism, 1985, p.31-32)

The area I have in mind is that of those attitudes and feelings, or "sentiments," as we used to say, toward ourselves and others, in respect of our and their actions, which can be grouped together under the heads of moral attitudes and judgments and personal reactive attitudes and are indissolubly linked with that sense of agency or freedom or responsibility which we feel in ourselves and attribute to others.

The fundamental thought is that once we see people and their doings (including ourselves and our doings) objectively, as what they are, namely as natural objects and happenings, occurrences in the course of nature

Again, neither determinism nor chance can provide free will

— whether causally determined occurrences or chance occurrences — then the veil of illusion cast over them by moral attitudes and reactions must, or should, slip away. What simply happens in nature may be matter for rejoicing or regret, but not for gratitude or resentment, for moral approval or blame, or for moral self-approval or remorse.

Attempts to counter such reasoning by defending the reality of some special condition of freedom or spontaneity or self-determination which human beings enjoy and which supplies a justifying ground for our moral attitudes and judgments have not been notably successful; for no one has been able to state intelligibly what such a condition of freedom, supposed to be necessary to ground our moral attitudes and judgments, would actually consist in.

Such attempts at counter-argument are misguided; and not merely because they are unsuccessful or unintelligible. They are misguided also for the reasons for which counter-arguments to other forms of skepticism have been seen to be misguided; i.e. because the arguments they are directed against are totally inefficacious. We can no more be reasoned out of our proneness to personal and moral reactive attitudes in general than we can be reasoned out of our belief in the existence of body.

A few years after Strawson's Naturalistic arguments for the moral sentiments he called the reactive attitudes, Willard van Orman Quine argued that epistemology should be naturalized.

In his essay Two Dogmas of Empiricism, Quine argued that the distinction between analytic (a priori) and synthetic (a posteriori) knowledge was moot because ultimately the "truth" or validity of analytic statements depended on their applying in the world.

Naturalized epistemology has been called "scientism" because it makes science the last word on whether we know what we think we know. And Quine initially agreed with Bertrand Russell that "what science cannot discover, mankind cannot know." Epistemological naturalism today assumes that science is the final arbiter of public knowledge arrived at by consensus of the community of inquirers. This was Charles Sanders Peirce's idea of pragmatic knowledge. But it also admits some private knowledge that may be unsuitable for such public empirical verification.

We should mention here the Center for Naturalism, who deny that individuals have ultimate responsibility for their actions (in the sense of origination - being the self-caused authors of their actions) and assert that free will is an illusion. Nevertheless, the Center believes that individuals should be held morally responsible for their actions, and should be given appropriate rewards or sanctions, to help control behavior. So their moral responsiblility position is similar to that of David Hume, and perhaps to John Martin Fischer's semicompatibilism, although Fischer is agnostic on the free will question, and Hume's free will is compatible with determinism.