The Main Principles of David Hume's Epistemology as a Source of Contemporary Positivism
Man as a Measure of Knowledge and Reality
David Hume's skeptical conclusions were intended to demolish metaphysical philosophy. And this was not only a rejection of the previous metaphysical systems, but a radical negation of the main metaphysical problems as meaningless. Substance, causation, necessity, problems connected with the nature of the Soul and God, were proclaimed by David Hume as false and unreasonable. "To burn every book, which is not based on the ground of human experience or pure mathematics. . ." This point of view looks so contemporary, so anti‑metaphysical. But according to David Hume, first of all, it means that all sciences, in spite of all the differences between their subjects and methods, are a specific reflection, not of the nature of external things, but only of the nature of human cognizing ability. "There is no question of importance, whose decision is not compriz'd in the science of man; and there is none, which can be decided with any certainty, before we become acquainted with that science. In pretending therefore, to explain the principles of human nature, we in effect propose a complete system of the sciences, built on a foundation almost entirely new, and the only one upon which they can stand with any security." 
And this sole "security ground" of the sciences, in fact, denies the possibility of scientific knowledge concerning something independent, necessary and objective in comparison with human consciousness. Starting from John Locke's empirical position: all human knowledge is gained from experience; Hume radically denied the rationalists' theories about innate ideas and principles. Every object can exist only in human sense experience and be cognizable only through it.
Hume's explanation of the contents of human sense experience is based on phenomenalistic principles. Perceptions are not immediate knowledge about external things affecting our sensations, but a thoroughly subjective sphere of consciousness. No external thing is allowed to cross the border of human experience. Perceptions and memories about them are the only possible objects of knowledge.
The distinction between impressions and ideas, which Hume made, and the investigation of their relation, have great philosophical importance. Ideas (simple or complex) are regarded as no more than an adequate reflection of simple or complex impressions. That is why we have the admission that the supra‑empirical, rational ideas of the Rationalists are unprovable dogmas.
The simple impressions are particular sensations we have of colors, tastes, surfaces, temperatures; the complex impressions are immediately perceived things, an apple, a dog, the sun, a tree. And ideas are nothing more than particular memories of those impressions which have already been experienced.
Each simple idea necessarily corresponds to one simple impression. As far as the impressions, the ideas, exist only in human understanding, one phenomenon represents another phenomenon in our own experience. This is illustrated by Hume with a simple and clear example; the idea of the color red, which is formed in the dark, and the impression which is produced in my eye when the sun is shining, are distinguished from each other only in degree, but not by nature. This holds even for complex ideas which do not correspond adequately to any complex impressions.
That is why from David Hume's point of view, it is impossible to find any cognizable contents which have not been received by experience. Impressions and the memories of these impressions, and their mental combinations, that is as far as human knowledge can reach.
On these grounds, David Hume posits two systems of reality: the firstthe reality of perceptions and memory, and the secondthe reality of the mind. The mind is able, on the basis of its judgments, to compose a picture of the Universe in spite of the fact that it never perceives all its parts. But as the "first" and also the "second" system of reality exist only in our sense experience, they do not say anything certain about the nature of the external world, the existence of which even remains problematic.
David Hume is quite right in his adverse criticism of the epistemological theory of John Locke. If we accept that every immediate object exists as a complex idea only in our experience, it will, in fact, be impossible to prove that our knowledge can represent the qualities and essences of external things.
These theoretical difficulties were explicitly mentioned by John Locke himself, but he could not deny that one can know something about external things. That is why he tried to find such sense qualities which necessarily belong both to external things and to internal ideas. These are, first of all, so‑called "primary qualities": size, forms, structure. . .
David Hume makes a point on the psychological impossibility of perceiving something without so‑called "secondary qualities": color, taste, noise, smell . . ., which Locke suggested were only subjective impressions in our minds.
But this criticism of Locke does not lead Hume to the most ordinary and obvious conclusions; the very objects of our perceptions are not our own sensations but those independent external things which affected our senses and made possible our perceptions.
How we are able to see colors and to hear sounds is another question when the objective causality of these qualities are only luminous and phonetic waves. But the important epistemological question is what must be regarded as the immediate object of our knowledge: our own sense perceptions or material existences which are independent of our minds.
Hume's solution of Locke's contradiction is based on the ground of subjective skepticism. External material reality is excluded from the sphere of our knowledge. And we are not allowed even to say whether objective things or the activity of our minds are the real causes of our sense experiences.
Our perceptions and our memories of themthese are the final reality which can be approached by our knowledge. But this is the real meaning of Berkeley's principle "esse is percipi".
This principle leads Berkeley into solipsism and compels Hume to say that human reason will never be able to explain the relation between our ideas and the external world.
But this entails an absolute contradiction between human knowledge and human practice, making it impossible to find a truly knowable nature.
Down with Substance!
The decision, that all possible objects are impressions or ideas which could not themselves exist independently and necessarily, presented David Hume with great theoretical difficulties.
The first objection is in connection with the possibility of objective reality and human experience; for how would we be able to perceive and even think about something, if it did not possess the main features of external reality: if it were not to exist in space, during time in the past, present and future?
The philosophical problems connected with the identity of objects are, in fact, insoluble from the standpoint of skeptical subjectivism. If the most simple and the most complex objects are only sense perceptions, every sense quality should be regarded only as a quality of our perception and never as a quality of any objective thing. But then how can we be sure that our combining of our own sensations with one object will be the same in different places and at different times, and also the same as other people's experience?
Hume tried to explain the identification of definite sense qualities with particular objects as a result of repetitive perceptions founded on similar spatio‑temporal positions. But this explanation could not give us the answer even to such a question as: Why cannot only I, but also other people, whose "past" experience is quite different in comparison with mine, perceive apples, dogs, crocodiles . . ., as connected with the same perceptions?
Also, if our minds are creating these associations, through which only the perceiving of things is possible, how would it be possible to have some sense experience on which ground these associations are formed? In other words, how was "past" experience possible?
Unable to give us a satisfactory answer to such questions, Hume changed his philosophical analysis simply by saying that there is nothing more remarkable than these ordinary things in our thoughts and reasonings.
In his Essay John Locke regarded every object in our experience as a complex idea of substance. This is a psychological product of the connection of so‑called "primary" and "secondary" qualities. And although the idea of substance cannot represent this unknowable external substance of material things, nevertheless it is formulated in connection with the objects of sense experience as the unique identity of their qualities.
David Hume has no need for substance even in these senses. It is not a rational idea but only a psychological illusion, only our imagination, when we apply our sense perceptions to one thing which does not exist anywhere. The idea of substance is only a name applied to a psychological complex of our impressions. Therefore, languagebut not the thing itself, and not even reasonis the creator of the unity and identity of things.
This epistemological thesis of Hume's has great philosophical implications. In this same sense Russell writes: "I wish to suggest that, wherever there is, for common sense, a 'thing' having the quality C, we should say, instead, that C itself exists in that place, and that the 'thing' is to be replaced by the collection of qualities existing in the place in question. Thus 'C' becomes a name, not a predicate.”  That is why the basic contents of human knowledge are not "particular things", but our "sense‑data" (Russell), "atomic propositions" (Wittgenstein), or "protocol propositions" (Carnap).
Therefore, the final reference of all scientific and philosophical propositions is not an independent, material reality, but only our subjective sense perceptions, the ultimate immediate contents of human experience. And J. O. Urmson is quite correct when he writes: “. . . the verification principle as involving a corresponding reformulation of Hume's epistemological thesis, that all meaningful words must be ostensively definable.” 
But it is quite clear that for human knowledge and practice it is not enough if we are only able to register our own sense‑data (red, sweet, hard . . .) or "atomic propositions" ("This is red"). The development of science and social practice mean not only the improvement of language but first of all the improvement of our knowledge concerning that which is independent of our senses and minds, material, substantial reality.
Hume's empirical critical explanation directed against metaphysical views about substance, as the unknowable substratum of the qualities of things, played a very important role in the history of philosophy. Substance is indeed quite meaningless if it is regarded as something beyond individual variations of concrete things. But Hume goes on: we are not only unable to prove the ontological existence of substance, we are unable to have any idea about it, because in our sense experience we have no impression of which such an idea could be a copy. And also we cannot discover any common essence, because we can perceive only particular things, but never something general.
In connection with these views, Hume found a very important feature of the whole of preceding philosophy, which is based on the rationalistic solution of the problem of substance: an impermissible logical jump from our immediate sense experienceswhich are constantly subjective perceptual feelingsto the objective, independent, external substantial existence of things themselves.
But not only and not primarily is this rationalistic, dogmatic concept of substance the object of Hume's and a subjective‑empiristic refutation of substance. Bertrand Russell is quite precise: "But now, the majority of us do not recognize substance as a useful concept. . . . "
The positivistic refusal of the philosophical value of substance is a denial that knowledge can prove the possibility of material reality at all; and Berkeley was more consistent than Hume when he regarded the problem of substance in such a sense. But Berkeley's obliteration of material substance by means of very strong anti-dogmatical arguments does not mean the obliteration of all substance, for Berkeley, on the other hand, quite dogmatically presupposed the substantial existence of the Soul and of God.
Hume's much more consistent and independent mind refused not only the possibility of knowledge about material substance as external self-existence but also the substantial nature of the Soul and of God.
As a result of his obliteration of the substantial difference between Soul and matter, David Hume thinks that objects can exist, and nevertheless can be nowhere, and also that we have to accept the existence of some “spatial perceptions”.
This is an unexpected and strange view, one never advanced in all previous philosophical tradition. For it was accepted (especially after Descartes) that, in so far as something has got spatial features, it must be accepted as a self‑existing material thing. But Hume's "spatial perceptions" could not be regarded as something independent and external from our minds. They are only perceptions, that is to say, the objective contents of somebody's experience. Therefore, there are such things which are not material, but space‑existing, and also there are such mental things which have spatial existence and are in this sense non-spiritual.
This is the theoretical basis of Hume's radical conclusions about the Soul and God. Their immortality and even existence are insoluble. But if Hume's refusal of the possibility of proving the substantial existence of the self can be used in the long run in understanding the nature of human consciousness, his refusal at the same time of the possibility of proving the substantial existence of matter is full of great theoretical difficulties.
Contemporary philosophy, indeed, must be an enemy of every transcendental concept of substance.
But if these aberrations are so deeply rooted in the very concept of substance, who would think of going back to it?
The scientific value of "substance" is most closely connected with the fact that truth is inseparable from reality.
If some one uninitiated in philosophy asked himself what it is that entitles us to consider something as real, the most probable answer would be: the fact that I can perceive it directly, that I can see it, touch it, smell it. . . .
There is much truth in this naive conception, but it must be defended from a great many objections, which are backed up by the most ordinary examples. Does not everyone dream and are not his dreams sometimes as vivid as direct perceptions? Are there not mirages and hallucinations? That is why we must first answer the question: what do we mean when we say that the things which we perceive directly really exist?
There is no better tried way to establish the objective existence of an object than to observe the manifestations of the results of the interaction between it and other objects. That which does not act and cannot be acted upon is not real.
Now, what makes things capable of interaction? To a great degree this question is equivalent to the question: what makes things real?
The answer could be the following: the fact that they possess certain properties. If an object is not characterized by dimensions, color, shape, structure, etc., it cannot effect even the slightest change in some other thing, and nothing can act upon it.
Is not a thing real in so far as it is the "sum" of unique properties, inherent to it alone? There is no doubt that it is impossible to find two objects absolutely identical. A difference in color, degree of hardness, shape, place, etc., can always be discovered in them. But if the thing is defined by the absolute specificity of its properties, it could not bear even the slightest alteration, because this would be a negation of the absolute necessity for this thing to possess these properties. The question becomes still more difficult if we review the entire "biography" of the objectthere are stages in it during which the object was characterized by properties that might be absent now.
There can be no doubt about the advantages of the structural explanation of the "thing" as a stable unity of properties. The basic task of structural analysis is to disclose the principle of the unity of the constituent elements of the object. The structure as a formal combination of the elements, as the law of their being linked together into a whole, eliminates to a certain degree the logical difficulties resulting from the substantialist explanation of the object. The structural conception of the "thing" as a specific relation between the whole and its parts does not need to transform the unity of properties into something which is beyond the properties and independent of them, into their hidden bearer, their metaphysical "support".
But the definition of the "thing" as a necessary system of qualities does not solve the question as to what unites these characteristicsa material, objectively ideal, or subjectively psychological principle. Russell, who wholly supports the structural definition of the "thing", feels compelled to note the insufficiency of this approach: "The analysis of the structure although complete will not tell you everything which you might want to know about the object. It will tell only what the parts of the object are like and how they are related to one another, but it will tell you nothing about the relation of the object to those objects that are not its parts or components." Therefore; in his opinion, one must resign oneself to the factual unknowability of physical things. We can affirm with certainty only that which concerns our immediate sensory experiences. "What can be said of the purely physical world is hypothetical, insofar as physics gives us no other information but about structure. "
A consistent materialist cannot be satisfied with such a "simple" solution of the problem. For him the "thing" exists and manifests itself in its relations with the environment. And if the principle of this reciprocal relationship is inaccessible to human knowledge, the "thing" itself and the essence of its structure certainly will remain unknowable.
To argue that the object can exist as a relatively independent thing capable of mutual relationships with other things presupposes the presence of general qualities in it, qualities that link it with other objects and make it belong to one reality.
The manifestation of specific and general characteristics is a necessary step for the disclosure of the connection of the object with its environment. But if we state the question more broadly, as a question of the principle of interaction in general, it becomes clear that we must find the most general characteristic inherent in all things, something that is common to all of them, i.e. their substance.
Since Plato's day philosophy has felt the dependence of logical knowledge on the acceptance of a universal essence of all that exists. All laws of nature, even when their effect is limited within the framework of a given kind of phenomenon and are valid only under certain conditions, always have a universal meaning for this kind of phenomenon and for these conditions. Heat makes all bodies expand, the earth attracts all falling bodies towards its center, all things have a cause. . . . If these statements did not contain the word "all", the law would not be a law, there would not be any natural or social necessity, no scientific forecast, and planning would not be possible. Human knowledge would have been eternally imprisoned in the maze of concrete facts and would not have passed from the description of the facts to their interpretation.
The Illusoriness of Causation
David Hume himself regarded as the main contribution of his philosophy the analysis of the causal relation, which he had made in his Treatise of Human Nature. The influences of this theory are historic.
It is well known, that for Immanuel Kant Hume's conclusion for the non‑analytical character of causality was the immediate occasion to awake him from his "dogmatic slumber", and that provoked his critical thinking.
The particular position of the causal relation may indeed be a ground for solving the most important philosophical problems.
The definition of causality as a principle innate in our minds has played a very important role in rationalistic proofs of reality and truth. This principle was the very basis of Descartes' escape from the subjective existence of the mind to the objective existence of God and a material world. If the cause is necessarily connected with the effect, then these two events must be independent self‑existences and not only ideas in our mind.
David Hume agreed that the causal relation between things meant a necessary connection between one thing, the effect, and another thing, a former event, the cause: "Two objects may be considered as placed in this relation, as well when one is the cause of any of the actions or motions of the other, as when the former is the cause of the existence of the latter."
Hume also thought that if causality is a real connection between objects it will be the only way from our sense impressions to external reality, although he and Ludwig Wittgenstein quite rightly connected the acknowledgment of a real causal relation between things, with an acknowledgment of all natural necessity. "If there were a law of causality, it may be put in the following way: there are laws of Nature.” 
Hume's subjective phenomenalistic interpretation of human experience makes impossible any knowledge about any real necessary connection between things. For if every particular object of our sense experience is only an impression in our mind, there could be neither "cause" nor "effect", because they cannot be in a position of necessary dependence on each other. In our experience we are able to find only spatio‑temporal connections of simultaneity and consistency of perceiving things, but never the action of engendering. One impression could not breed any other. The phenomena in our sense experience are discrete, and we shall never be able to discover necessary connections between them. Causal necessity is also unattainable in a purely logical way, because, as Hume fairly discovered, there is no such thing as a deductive conclusion from cause to effect. But what remains from Hume's phenomenalistic position is a psychological solution only. Causation is discovered as a mental association, producing the illusion of necessary connections between perceived things, when such connections nowhere exist. But this illusion at the same time is the very basis of our expectations, that in the future, too, as well as in the past, the events of our experience will be given in the same order. In this respect, the development of Hume's views, as well as the theories of contemporary positivism, are a negation of the cognizable ability of human reason. The meaning of scientific laws is connected not with the real nature of things, but with our behavior, based on the ground of our belief that what we experienced yesterday will be experienced tomorrow too. Moritz Schlick says that the word "causation" has no theoretical meaning. And in so far as scientific laws are based on causal relations, the laws of nature could not be statements which could be true or false.  But if such is the case, in spite of our reason we are in no better position than the animals, whose behavior is also based on their habits and past experiences.
For Bertrand Russell, also, belief as a result of often repeated similar experiences is the common basis of human beings' and of animals' behavior. "Belief, as I wish to use the word, denotes a state of mind or body or both, in which an animal acts with reference to something not sensibly present. When I go to the station in expectation of finding a train, my action expresses a belief. So does the action of a dog excited by the smell of a fox. So does that of a bird in a room, which flied against the window panes in the hope of getting out. Among humans, the only action by which a belief is expressed is, very often, the pronouncing of appropriate words." 
But human knowledge in general and human science in particular need much more definite criteria about truth and reality than animal belief.
"Belief" for Hume means a psychological act which creates reality, or what is presupposed to be reality, and for Russell, too, what remains for us in the end is only to believe in a certain correspondence between our ideas and some features of external reality. ". . . The external reference of an idea or image consists in a belief, which when made explicit, may be expressed in the words: 'this has a prototype'. In the absence of such a belief (which, when it exists, is usually a somewhat vague feeling) although there may be in fact a prototype, there is no external reference. This is the cause of pure imagination." 
But if this "belief" (which, as Hume said, must be "joined" to the ideas of judgments, and "does not exist" in the fiction of our imagination), "makes" the reality, such "reality" is not in fact reality, because it could not be something independent of our consciousness nor even of our feelings. This turning back into "belief", in connection with the definition of truth and reality, exposes the theoretical weakness of Hume's and the positivists' philosophical views. It is not, indeed, a belief in ghosts and spirits, but a psychological belief which applies to reality and knowledge, such as Hume's, is not substantially different from a belief in God, because both are rationally unprovable.
Hume made his main logical conclusions on the basic principles of subjective empiricism, and more than anybody else he discovered its theoretical helplessness. Russell is very precise when he writes, that Hume "represents in a certain sense a dead end: in his direction it is impossible to go further."
However, this objective estimation is at the same time the gravest verdict against all contemporary positivism, which has "reformulated" the main principles of Hume's philosophy, but has nothing new to say about the solution of cardinal philosophical problems except: "And so it is not wondered at that the deepest problems are really no problems. . ."  "Whereof one cannot speak, thereof one must be silent,"  as Wittgenstein said in his Tractatus.
1 David Hume, A Treatise of Human Nature, Oxford University Press, 1965, p. xx.
2 Bertrand Russell, An Inquiry into Meaning and Truth, 1948, p. 98.
3 J. O. Urmson, Philosophical Analysis, Oxford University Press, 1956, p. 111.
4 Ludwig Wittgenstein, Tractatus Logico‑Philosophicus, 6.36.
5 Moritz Schlick, "Causality in Everyday Life and in Recent Science", Contemporary Philosophic Problems, New York, 1959, p. 357.
6 Bertrand Russell, Human Knowledge, 1948, p. 129.
7 Ibid., p. 125.
8 Bertrand Russell, The History of Western Philosophy, London, 1948, p. 685.
9 Tractatus Logico‑Philosophicus, 4.003.
10 Ibid., 6.57.
SOURCE: Panova, Elena. “The Main Principles of David Hume's Epistemology as a Source of Contemporary Positivism,” in: Revolutionary World: An International Journal of Philosophy (Amsterdam: B. R. Grüner B. V.), vols. 11: 12: 13, 1975, pp. 218-227.
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