The trend toward using natural language in English philosophy that began with Roger Bacon came to a head with David Hume. His matter-of-fact approach was like a dash of cold water on the heated controversies of his day, perhaps a needed dash of cold water. The heated harangues of the schoolmen having been replaced by the technical obscurity of the rationalists, philosophy was rapidly becoming irrelevant to both life and the new Newtonian science. The works of Locke were highly influential during this period. But few people really understood them. Above all Hume championed a return to something approximating common sense. His attempt to clarify and simplify the complex logic of John Locke was much better understood and had an immediate impact on the eighteenth century world. Hume thus began "An Enquiry Concerning Human Understanding" by describing the two approaches philosophers take to moral philosophy.

Moral philosophy, or the science of human nature, may be treated after two different manners; each of which has its peculiar merit, and may contribute to the entertainment, instruction, and reformation of mankind. The one considers man chiefly as born for action; and as influenced in his measures by taste and sentiment; pursuing one object, and avoiding another, according to the value which these objects seem to possess and according to the light in which they present themselves. As virtue, of all objects, is allowed to be the most valuable, this species of philosophers paint her in the most amiable colors; borrowing all helps from poetry and eloquence, and treating their subject in an easy and obvious manner, and such as is best fitted to please the imagination and engage the affections.

The other species of philosophers consider man in the light of a reasonable rather than an active being, and endeavor to form his understanding more than to cultivate his manners. They regard human nature as a subject of speculation; and with a narrow scrutiny examine it, in order to find those principles, which regulate our understanding, excite our sentiments, and make us approve or blame any particular object, action, or behavior. They think it a reproach to all literature that philosophy should not yet have fixed, beyond controversy, the foundation of morals, reasoning, and criticism; and should forever talk of truth and falsehood, vice and virtue, beauty and deformity, without being able to determine the source of these distinctions. While they attempt this arduous task, they are deterred by no difficulties; but proceeding from particular instances to general principles, they still push on their inquiries to principles more general, and rest not satisfied till they arrive at those original principles, by which, in every science, all human curiosity must be bounded.

The description soon becomes an invective for he went on to say, "Though their speculations seem abstract, and even unintelligible to common readers, they aim at the approbation of the learned and wise; and think themselves sufficiently compensated for the labor of their whole lives, if they can discover some hidden truths, which may contribute to the instruction of posterity." The more obvious philosophy, he said, is to be preferred by the general mankind, it enters more into the common life. The more abstruse philosophy which cannot enter into business and action "...vanishes when the philosopher leaves the shade and comes into open day." Therefore it is the easy philosopher that is the most durable and which gains the greatest fame, the others enjoying only a momentary reputation. But he lists as those who have retained their fame, Cicero, La Bruyere, and Addison and those who have been forgotten, Aristotle, Locke and Malebranch. Interestingly enough, if one were to ask the common man today to name two philosophers, he might name Plato and Aristotle, but he would confess that he knows absolutely nothing concerning either. However, he has struck his colors and laid out his plan of attack and it is this attack more than his philosophy that has earned a place in the history of ideas for David Hume. As was normal in his approach to philosophy. he put that plan of attack in simple unequivocal terms.

The sweetest and most inoffensive path of life leads through the avenues of science and learning; and whoever can either remove any obstructions in this way, or open up any new prospect, ought so far to be esteemed a benefactor to mankind. And though these researches may appear painful and fatiguing, it is with some minds as with some bodies, which being endowed with vigorous and florid health, require severe exercise, and reap a pleasure from what, to the generality of mankind, may seem burdensome and laborious. Obscurity, indeed, is painful to the mind as well as to the eye; but to bring light from obscurity, by whatever labor, must needs be delightful and rejoicing.

But this obscurity in the profound and abstract philosophy, is objected to, not only as painful and fatiguing, but as the inevitable source of uncertainty and error. Here indeed lies the justest and most plausible objection against a considerable part of metaphysics, that they are not properly a science; but arise either from the fruitless efforts of human vanity, which would penetrate into subjects utterly inaccessible to the human understanding, or from the craft of popular superstitions, which, being unable to defend themselves on fair ground, raise these entangling brambles to cover and protect their weakness.

The only method of freeing learning, at once, from these abstruse questions, is to enquire seriously into the nature of human understanding, and show, from an exact analysis of its powers and capacities, that it is by no means fitted for such remote and abstruse subjects. We must submit to this fatigue in order to live at ease ever after. And must cultivate true metaphysics with some care in order to destroy the false and adulterate.

The first point that Hume made in his inquiry dealt with simplifying the Lockean concept of the origin of ideas. All perceptions of the mind, he said, could be simplified into two species, thoughts or ideas, and impressions. By impressions he meant what occurs in our minds when we see, hear, feel, love, hate, desire, or will. By thoughts and ideas he meant what occurs in our minds when we reflect on any of the above. This simplification does away with the problem of Locke's primary and secondary qualities. Qualities are attributes of things external to the mind. Ideas are not. The mind can only deal with ideas. He said that we form thoughts of things that do not or cannot exist by concatenating thoughts of things we have experienced. In his words, "...all this creative power of mind amounts to no more than the faculty of compounding, transposing, augmenting or diminishing the materials afforded to us by the senses and experience." He said that if we analyze our thoughts or ideas we find that they resolve themselves into simple ideas copied from a precedent feeling or sentiment. "The idea of God", he said, "as meaning an infinitely intelligent, wise, and good being, arises from reflecting on the operations on our own mind, and augmenting without limit, those qualities of goodness and wisdom." But what has this done with faith? He claimed that this did not involve a rejection of faith. His complaint was against the various ontological proofs of the existence of God put forward by the rationalists and others. Later, answering to a charge of impiety, he put forward a form of the argument from design, not as a proof of the existence of God, but as a reason for faith. Still, it would be difficult for anyone to justify even this faith once one accepts the idea that metaphysical statements have no meaning. A century and a half later this idea was to gain popularity through the efforts of a group of philosophers called the Vienna Circle. From when Aquinas began introducing reason into religion, this tendency to divorce philosophy from religion has now achieved its full potential and religion has been reduced to myth and superstition.

If a defect of an organ has prevented a person from experiencing a sensation then he can have no idea of the sensation. For example a blind man can have no idea of color nor a deaf man of sounds. Hume said that if the following simple proposition were adhered to then metaphysics could be cleared of the senseless jargon that has drawn disgrace on it.

All ideas, especially abstract ones, are naturally faint and obscure: The mind has but a slender hold of them: They are apt to be confounded with other resembling ideas; and when we have often employed any term, though without a distinct meaning, we are apt to imagine it has a determinate idea annexed to it. On the contrary, all impressions, that is all sensations, either outward or inward, are strong and vivid: The limits between them are more exactly determined: Nor is it easy to fall into error or mistake with regard to them. When we entertain, therefore, any suspicion that a philosophical term is employed without any meaning or idea (as is but too frequent), we need but inquire, from what impression is that supposed idea derived? And if it be impossible to assign any, this will serve to confirm our suspicion. By bringing ideas into so clear a light, we may reasonably hope to remove all dispute, which may arise concerning their nature and reality.

Once we have determined that the ideas that we are examining have a determinate meaning we must next consider the relations between them. There are only three ways, Hume said, that ideas could be associated, resemblance, contiguity in time and place, and cause or effect. Of the objects of human reason we can divide them into two kinds, relations of ideas and matters of fact (often called Hume's Fork). Of the first kind he meant anything that is intuitively certain such as the sciences of geometry, algebra, and arithmetic. As an example he said that the Pythagorean theorem that the square of the hypotenuse is equal to the square of the two sides is a proposition which expresses the relation between these two ideas. As Hume put it, "Though there were never a circle or triangle in nature, the truths, demonstrated by Euclid, would forever retain their certainty and evidence." But matters of fact are something quite different.

Matters of fact, which are the second objects of human reason, are not ascertained in the same manner; nor is our evidence of their truth, however great, of a nature like the foregoing. The contrary of every matter of fact is possible; because it can never imply a contradiction, and is conceived by the mind with the same facility and distinction, as is ever so conformable to reality. That the sun will not rise tomorrow is no less intelligible a proposition, and implies no more contradiction, than the affirmation, that it will rise. We should in vain, therefore, attempt to demonstrate its falsehood. Were it demonstrably false it would imply a contradiction, and could never be distinctly conceived by the mind.

The point that Hume was making is that all ideas concerning matters of fact come only from experience and never from reasoning. Particularly important is that causes and effects are discoverable only through experience and not through reasoning.

This proposition, that causes and effects are discoverable, not by reason, but by experience, will readily be admitted with regard to such objects, as we remember to have been once altogether unknown to us; since we must be conscious of the utter inability, which we then lay under, of foretelling, what would arise from them. Present two smooth pieces of marble to a man, who has no tincture of natural philosophy; he will never discover, that they will adhere together, in such a manner as to require a great force to separate them in a direct line, while they make so small a resistance to a lateral pressure. Such events, as bear little analogy to the common course of nature, are also readily confessed to be known only by experience; nor does any man imagine that an explosion of gunpowder, or the attraction of a lodestone, could ever be discovered by arguments a-priori. In like manner, when an effect is supposed to depend upon an intricate machinery or secret structure of parts, we make no difficulty in attributing all our knowledge of it to experience. Who will assert, that he can give the ultimate reason, why milk or bread is proper nourishment for a man, not for a lion or tiger.

However, this does not answer the question concerning how we do gain knowledge from experience.

When a man says, I have found in all past instances, such sensible qualities conjoined with such secret powers: and when he says, similar sensible qualities will always be conjoined with similar secret powers; he is not guilty of a tautology, nor are these propositions in any respect the same. You say that the one proposition is an inference from the other. But you must confess that the inference is not intuitive. Of what nature is it then? To say it is experimental, is begging the question. for all inferences from experience suppose, as their foundation, that the future will resemble the past, and that similar powers will be conjoined by similar sensible qualities. If there be any suspicion that the course of nature may change, and that the past may be no rule for the future, all experience becomes useless, and can give rise to no inference or conclusion. It is impossible, therefore, that any arguments from experience can prove this resemblance of the past to the future; since all these arguments are founded on the supposition of that resemblance.

What is the principle by which the understanding arrives at a conclusion concerning the relationship between a cause and its effect? Suppose, Hume suggested, that a person endowed with the strongest faculties of reason and reflection were to be brought into this world. Though he would witness a continual succession of objects, he would not be able to discover anything further. He would never be able to discern the idea of cause and effect because the particular powers by which all natural operations are performed never appear to the senses. There would be no reason to infer from the existence of one object the appearance of another. Without experience this person could never employ his reasoning concerning any matter of fact to determine any other fact beyond what is present to his senses and his memory.

Suppose again, that he has acquired more experience, and has lived so long in the world as to have observed similar objects or events to be constantly conjoined together; what is the consequence of this experience? He immediately infers the existence of one object from the appearance of the other. Yet he has not, by all his experience, acquired any idea or knowledge of the secret power, by which the one object produces the other.

The principle that produces in him the confidence that in every case the occasion of one object is followed by the occasion of the other is custom or habit. By custom Hume meant a principle of human nature by which we decide that we have reached sufficient satisfaction, that to continue our enquiries would carry us no further. This is why, Hume said, we draw from a thousand instances, an inference, which we are not able to draw from one instance that is in no respect different from them.

Custom, then, is the great guide of human life. It is that principle alone, which renders our experience useful to us, and makes us expect, for the future, a similar train of events with those which have appeared in the past. Without the influence of custom, we should be entirely ignorant of every matter of fact, beyond what is immediately present to the memory and senses. we should never know how to adjust means to ends, or to employ our natural powers in the production of any effect. There would be an end at once of all action, as well as the chief part of speculation.

The conclusion of this reasoning is that "All belief of matter of fact or real existence is derived merely from some object, present to the memory or senses, and a customary conjunction between that and some other object." The sentiment of belief is nothing more than a conception that is more intense than one of fiction and that it is derived from the constant conjunction between the object and something present to the senses or the memory.

When it comes to moral sciences there are further problems. The geometrical sciences have the advantage that the ideas are determinate, but the problem is that mathematical reasoning requires long and often arduous steps, each step clear and distinct. Moral reasoning, on the other hand, requires shorter trains of reasoning but the terms are often vague or arbitrary.

The chef obstacle therefore to our improvement in the moral or metaphysical sciences is the obscurity of ideas, and ambiguity of the terms. The principle difficulty in mathematics is the length of inferences and the compass of thought, requisite to the forming of any conclusion. And, perhaps our progress in natural philosophy is chiefly retarded by the want of proper experiments and phenomena, which are often discovered by chance and cannot always be found when requisite, even by the most diligent and prudent enquiry. As moral philosophy seems hitherto to have received less improvement than either geometry or physics, we may conclude that, if there be any difference in this respect among these sciences, the difficulties, which obstruct the progress of the former require superior care and capacity to be surmounted.

This is the basic problem of the justification of true propositions, though as put by David Hume, it is simply the logical independence that exists between belief and reality. However, this assumes that knowledge exists as a set of propositions, principles, facts, laws, etc., and an identity between information and knowledge that has never been shown to exist.

If we attempt to compare what we might call the "Humean" problem with what Aristotle called Plato's Hericlitean problem we find some interesting sidelights. For Plato that the world was rational was a basic assumption, but he avoided the problem of cause and effect. He could do this because knowledge, being simply recollection, did not apply to the changing world of experience. However, the idea that the world is rational cannot be determined through experience. It is, and has always been an assumption upon which all of western culture is based. If there is a fallacy behind Hume's thinking, it is that he is using reason to demonstrate the futility of such assumptions. But without the assumption reasoning itself is futile. In an irrational universe nothing can be implied by anything else. Has Hume then, by deriving all knowledge only from experiencel built a box from which there is no exit? The answer to that question is not easy. On the other hand, the Humean problem served for philosophers following him, the same purpose that the Hericlitean problem served for those who followed Hericlitus. Modern philosophers, operating from the same assumptions that David Hume operated from found that before they could proceed beyond Hume, they were forced to either solve his problem, or avoid it, just as Plato and his followers were forced to solve or avoid the Hericlitean problem