A New Philosophy of Experience

by Moritz Schlick

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In the First Lecture, which I had the pleasure of delivering here a few months ago, I tried to give a short sketch of the nature and method of philosophy; and I spoke about the position which I think philosophy will take in the future after its real nature has been more generally and more properly understood.

Today I shall try to outline the results of a consistent application of the true method of philosophy to one or two of the great traditional problems. There are different ways of approaching philosophy, but the most natural one is to start from some fundamental issue around which all the other problems seem to group themselves in a systematic order.

Such a central problem with which I should like to begin is the question, “What can we know?” It is a truly fundamental issue. Kant spoke of this question as one of the three great questions which metaphysics has to answer. No other problem causes such a sharp division between the various schools of philosophy and the answer given to this question characterizes the philosophical systems and mental attitudes better than anything else. We find within ourselves a thirst for knowledge, a desire to explain, a craving for answers to endless questions; and every one who thinks has some moments in his life when he asks himself. “Can this thirst be quenched at all? Can this desire for knowledge be satisfied; and if so, how far can it be satisfied?” In other words, the problem seems to be, “What questions can be answered?”

There are two extreme positions which can be taken in regard to this question. One would be to answer, “We cannot know anything; no questions can be finally answered.” And the other one would be to say, “We can know everything, and there is no question which cannot finally be answered by the human mind.” The first of these attitudes is called skepticism, and the second one would be called, perhaps, dogmatism. The skeptic doubts everything, and the dogmatist does not suffer his fundamental beliefs to be touched by any doubt. Neither of these two philosophies, the skeptical or the dogmatic, really needs to be taken too seriously. The skeptic also believes that he can answer a good many questions, at least in every day life, and he really has no serious intention of denying absolutely everything; and the dogmatist, we may be sure, cannot help but feel certain doubts in his mind and in his heart. He does not really believe that everything can be answered satisfactorily.

Most philosophies, therefore, take their position somewhat between those two extremes. They do not assert that all questions can be answered and they do not believe that no problem can be solved. They all believe that a certain boundary exists between those problems to which we can find the final clues and those for which a solution seems to be forever impossible. The place where they draw the boundary line between that which can be known and that which cannot be known is, as I hinted before, one of the most characteristic traits of different philosophies.

Thus most philosophers believe that there is the knowable and the unknowable, that there are answerable and absolutely unanswerable questions. And this seems to be the attitude not only of philosophers, but of all ordinary people, too. All of us, perhaps, believe that surely some questions can be answered, and that surely there are some questions to which we cannot find any solution.

But there is an important distinction to be made here. In order to see this, let us look at some questions of every day life, of science, of philosophy. I take several instances on different levels. Suppose we should ask, “When will the depression end?” None of us, I expect, knows the answer, but we have no doubt that in some future time the answer will be known. Most of us also believe that if there were some one who really knew all the facts and had the ability of drawing the proper conclusions he would be in a position to answer this question even now. This means that we do not regard such a question as unanswerable. We do not happen to know the answer, but we believe that the finding of it is in no way beyond human possibilities.

Take another question, perhaps a little more complicated. If the historian should ask, “What did Napoleon do on January 2, 1800 at 5:32 in the afternoon?” — it might happen to be known, but probably is not known; it is also possible that no historian as long as the human race exists will ever be able to answer the question definitely. Thus, from a certain point of view, this question may seem unanswerable. There may be no means, as far as our human possibilities go, of ever finding out what Napoleon did at that moment. But although in one sense of the word this question is perhaps unanswerable, we do not get excited about a problem of this kind; the impossibility, if it should be impossible to solve the problem, is not one of very serious nature, because it would not be an intrinsic impossibility. It always remains possible, e. g. that a document might be found which tells us what Napoleon did at that particular time, or from which it could be infered in some indirect way.

Let us quickly look at some other questions. Geologists ask, “What was the earth like a billion years ago?” There were surely no historians there to tell us about the state of the earth at that time, and if we can ever find it out it will have to be in some indirect way. We must draw conclusions from our knowledge of stars that have developed. By such observations, combined with our knowledge of the laws of nature, this question can be answered more and more definitely. We are able to make many reliable statements about the development of planets like the earth or about stars in general; and yet our present science has not developed far enough to tell us exactly what the state of the earth must have been a billion years ago. There may be a certain sense in which this question, too, may be unanswerable. For science will, perhaps, never get so far that it really can answer all the different questions which might be asked in regard to the state of the earth at that time.

Let us go on to some other question. “What is the substance of a distant star?” I take this particular question on purpose, because there was a French philosopher, August Comte, the exponent of Positivism, who expressed his belief that it could never be known of what substances distant stars consisted. Nowadays we have spectral analysis which allows us to make very definite statements about the chemical elements and their physical conditions which form the material of suns that are thousands of light-years distant from the earth. This is a good example of a problem that was pronounced insoluble by a leading philosopher, but was, only a short time afterwards, completely mastered by science.

Take another problem. “Is the universe finite or infinite in space and in time?” This used to be a typical philosophical question. It played a rather important part in Kant's philosophy. He had a series of arguments which he believed to be perfectly valid from the ordinary point of view and which proved that the universe must be finite in space and time; and he had another set of arguments on the opposite page which proved that the universe must necessarily be infinite in both respects. He believed that there was a real contradiction between these two proofs, which could be overcome only by his own philosophy with its distinction between the world as appearance and the world as reality. Again modern scince, discarding entirely Kant's merely speculative reason, has very definite arguments in favor of the view that the universe is finite in space. There are in the first place astronomical reasons which give a certain probability to this view, but there are also general reasons derived from the application of modern physics to problems concerning the structure of the universe which seem almost inevitably to lead to the conclusion that the universe must be finite in space, though probably not in time. The proof will rest on astronomical observations and our knowledge of the laws of nature and on nothing else.

This case is similar to the one of the former question in that the problem seems to be shifted from the realm of philosophical speculation to that of scientific observation, thereby changing from an apparently hopeless issue into a perfectly answerable question. With the development of our knowledge we come to see possibilities which formerly were not known to men; and therefore the number of insoluble problems seems to diminish. What is the significance of this process, and can it go on indefinitely?

Now let us consider another “purely philosophical” problem, one of the oldest in philosophy; the “relationship between body and mind.” Are there perhaps two substances in the world, physical and mental substance; and what is their interaction? Is there any such thing as mind, or must everything be explained in terms of physics? Is there any such thing as matter, or must everything be explained in terms of mind? Such are the questions about which philosophers have debated. Concerning this particular problem, not a few of them have taken the attitude that it is a good instance of an unanswerable question. They say, for instance, that there will never be the slightest hope of understanding even the simplest act of sensation.

They argue that it is impossible to imagine any way in which a certain physical process in the brain can be transformed into a sensation, i. e. into something mental. Well known writers have pronounced a definite ignorabimus in regard to the question how the gulf between mental and physical processes can be bridged, while others have thought they could get rid of the problem easily enough by way of some dogmatic metaphysics.

The chief problem of so-called metaphysics is supposed to be the question “What is the essence of Reality? Is the world essentially mental? Is it mind, as was taught by Berkeley, or is it matter as was taught by the materialists, or is it something which cannot be known, as Kant's ‘thing in itself’?” Thus, while most of the older philosophers seemed to have little difficulty in solving the problem, Kant maintained most emphatically that there was no solution to any metaphysical problem, i. e. any question regarding the “ultimate nature of reality.” And he did this although he did not deny that these questions had a perfectly good sense and that the human mind would always keep on asking them.

Just one more question with which to end up, and which is also typical. “Have animals consciousness?” Most of us think “Yes, of course; a dog is in many respect almost like a human being. How could this be if it did not have sensations and feelings?” Yet there has been a very enlightened philosopher, Descartes, who held the view that consciousness was something reserved for human beings, and that all animals were automata. Their behaviour should be regarded as mechanical reactions on certain stimuli without the intervention of any “consciousness.” As we can certainly never get into an animal in order to see if it has any feelings, it can surely never be decided by observation and experiment whether Descartes might not be right. And we may even go further and say that the same argument applies to human beings, for although we seem to know when we look into ourselves that we ourselves possess consciousness, how can we be sure about our fellow beings? Descartes thought there was a better argument for human beings possessing a soul than animals, because the existence of other consciousnesses is always infered by analogy, and there is a closer analogy between myself and other human beings than there is between human beings and animals. But, on the other hand, there is nothing ever really proved by analogy; so, if Descartes' argument leads to anything at all, it must lead to doubt concerning the existence of any consciousness of other beings, whether human or animal. And it seems that his doubt must remain forever, as it is generally admitted that it is absolutely impossible for us to become directly aware of any one else's consciousness. We cannot have any immediate knowledge of the mental states of others. There is no way of becoming immediately acquainted with any soul except our own, and so the problem of the existence of mental life in our fellow beings or indeed anywhere else seems to be a typical case of those questions the answers of which we shall never know.

Let us conclude this little survey and see what we can learn from it. There are certain questions among our examples of which we could say immediately that we are able to solve them; there are some others where this seems doubtful, and still others where it seemend quite necessary to admit that we shall never know the answer.

But in this latter case we must make a very important distinction. If we ask, for example, what Napoleon did at a particular minute of his life, it is very likely that nobody will ever know the answer. And yet such a question, as I have already pointed out, is not unanswerable in some very essential sense of the word. It just happens to be unanswerable for us because we do not possess any historical evidence concerning the facts; there is no present experience from which we could infer exactly what Napoleon did at that moment, but it is not unknowable in principle. It is just unknowable through accidental circumstances, not because of the intrinsic nature of things.

There is an enormous difference between these two kinds of impossibility. We must distinguish between those problems which are insoluble in principle, and those which man cannot solve because he does not happen to have the technical means necessary for the solution. Whenever he comes across an unanswerable question, the philosopher must ask himself to which of these two classes it belongs. It is likely that no human being will ever know what the back of the moon looks like, because the moon always turns only one and the same side towards the earth. But the impossibility of knowing the far side of the moon is only a practical or technical one; it is due to the fact that man has not yet invented a ship that will take him around the moon, but such an enterprise would by no means be contrary to the laws of nature, and even if it were against the laws of nature we might imagine the laws of the universe to be different in such a way that it would be no longer impossible for human beings to look at the back of the moon. So we see that our question certainly belongs to the first group; the reasons why it cannot be answered are merely of an accidental nature. This may interest the scientist, but the philosopher is not concerned with it, he is worried by the other group of problems: those which are insoluble in principle: the reason why they cannot be solved is not an accidental state of affairs in the universe, but it seems to lie deeper; in this second case we speak of a philosophical or logical impossibility.

The difference between the two groups is this, that in the first one we can at least imagine means of finding a solution, even if these means exist nowhere in the world, whereas in the case of philosophical impossibility no imagination can bring us nearer to the answer; there are no ways on which even imagination could try to reach the goal. We cannot imagine what we would have to do or what would have to happen in the world in order to lead us to the answer of our former question: “How can a sensation arise from motions of molecules of the brain?” This question, therefore, belongs to the second group; philosophers have always worried about it. In most philosophies there are problems of this kind: certain questions within them are believed to “pass our understanding” or to be mysteries which we cannot fathom.

Our “Philosophy of Experience” takes an entirely different attitude. In order to understand it let us ask: “What is the criterion by which we decide whether an ‘insoluble’ problem belongs to the first or to the second group?” I think the criterion must be stated in this way: All the questions that can in principle be answered (including those that may at any one time or place be technically insoluble) are always answerable in one way, namely by reference to some observation (be it of nature or of ourselves), or by any scientific method which always pre-supposes observation, i. e. the occurrence of some sense impressions — in short, by experience.

A question is in principle answerable (I should like to say: it is a “good question”) if we can imagine the experiences which we would have to have in order to give the answer. An answer to any question is always a proposition. In order to understand a proposition we must be able exactly to indicate those particular circumstances that would make it true and those other particular circumstances that would make it false. “Circumstances” means facts of experience; and so experience decides about the truth or falsity of propositions, experience “verifies” proposition, and therefore the criterion of the solubility of a problem is its reducibility to possible experience. We can know what is verifiable. A question is a “good” one if we can indicate the way to its verification by possible experience — although, for some practical reason, we may be unable to follow that way.

Before speaking about those problems that are held to belong to the strictly unanswerable kind (i. e. by some philosophers, for there is no general agreement on this point), let us ask if there are perhaps any answerable questions the criterion of which does not lie in experience? These would evidently go beyond the realm of experience; the propositions answering them would have to be verified in some other way, they would deal with facts outside and independent of experience.

Many thinkers believe that such problems and such solutions exist; the field beyond experience with which these questions and answers are concerned would be the field of “Metaphysics”, and the criterion which would assure us of the truth of those answers woud not be experience, but “reason.” The philosophers who believe that there are such truths which cannot be accounted for by experience but rest on reason, are called rationalists (ratio = reason), and it is natural for them to think that all the most fundamental philosophical truths are of this kind. Those who do not believe that We cannot have any real knowledge that is derived from our reason but maintain that it must always rest on experience are called Empiricists (empeiria = experience). We see from this explanation why the meta-physician is usually a rationalist at the same time, while the empiricist rejects the possibility of metaphysics, i. e. of any knowledge that would reach beyond the world of experience. It is true that in the history of philosophy we sometimes find intermediate points of view, so that the equations rationalist = metaphysician, empirist = non-metaphysician, are not quite correct historically, but those views are due to certain confusions which complicate the matter and with which we do not have to concern ourselves.

Usually both parties admit the existence of a certain boundary which walls in everything that is knowable by experience and separates it from the rest of the world. But the metaphysician believes that this wall can be scaled by our reason, while the empiricist believes this to be impossible and regrets it. Both usually believed that there were definitely unanswerable questions, and both found them in the metaphysical region. The difference was that the empiricist was convinced that all problems lying in that region were insoluble, while the rationalist believed he could solve the most important ones of them by means of his reason.

This issue between rationalism and empiricism went on continuously through the centuries, and the chief reason why rationalism was very often deemed to be victorious lay in the fact of the existence of what is called logical and mathematical truths. All the best thinkers from Plato’s time on recognized that these truths certainly did not rest on any experience, and yet nobody could deny that they not only were really true, but even the most firmly established truths of all, and without doubt applicable to reality. But if this were so, then there were certain questions (those of logical and mathematical nature) which could be answered without consulting experience, and our criterion which we thought could distinguish soluble problems from those that are in principle and definitely insoluble, would break down.

There is no time to explain here in full how much depends for philosophy on the decision of this issue, but it has always been felt by the deepest thinkers, and therefore they concentrated their whole energy on the discussion of the so-called logical and mathematical truths. They felt that almost everything in philosophy was decided, if they could understand the nature of those particular truths. Kant based his whole system on such an investigation, and he really believed that by it he succeeded in overcoming the dispute between empiricism and rationalism.

In reality he has not succeeded. His solution of the problem was just as unsatisfactory as that given by the empiristic school, for instance by its most famous leader in the nineteenth century, John Stuart Mill. He endeavored to prove that reason alone could not solve any problems at all and that the only test for the truth of any proposition lay in experience; he attempted to show that logical and mathematical propositions (such as 2 + 2 = 4) had no other reason for being true than that they were always found to be so in experience. But a critical examination of his argument reveals the most serious mistakes in it, and we must conclude that he failed utterly in his attempt to show the empirical nature of logical or mathematical propositions. In Kant’s time and in the nineteenth century the tremendous difficulties involved in this problem could not be clearly seen, and the older thinkers arrived at their results in rather a superficial manner, but when during the last two or three decades the problem of the nature of mathematical propositions presented itself to the mathematicians within their own territory and called for a definite solution for the sake of pure mathematics alone, it was taken up again by logicians empirically-minded philosophers, and mathematicians, under the leadership of the most subtle and critical of the last.

The work of all these thinkers has not yet been entirely completed — a few little gaps still have to be filled out — but there is not the slightest doubt any more what the final solution looks like. It can be expressed briefly in the following way: mathematical and logical propositions are so entirely different in nature from ordinary empirical propositions that perhaps it is even unwise to call them both by the same name. It is so difficult to realize this difference, and it has never been clearly seen before, because both are expressed in language by sentences of the same form. When I say: “The latitude of San Francisco is 38°,” and when I say: “The fifth power of 2 is 32,” both sentences not only seem to have a similar grammatical construction, but they also seem to impart some real information. The one speaks of a certain city in California in a similar way as the other seems to speak about certain numbers: how can there be any intrinsic difference between them?

Strict logical analysis has shown that there is the greatest imaginable difference. It is simply this, that empirical propositions really deal with something in the universe, they communicate actual facts, while mathematical propositions do not deal with anything real. “Numbers” cannot be found anywhere in the real world in the same way in which San Francisco can be found there. It is also not true that numbers are simply imaginations of the human mind, like dragons or angels, and it is not true that, together with other “unreal” objects, they belong, to a world of their own, which Plato called the realm of Ideas, and in which many present day philosophers also believe, giving different names to it. The fact is that numbers or other logical entities cannot be regarded as “objects” in any sense at all. The propositions apparently dealing with them do not communicate any “facts” about them and therefore are no proper “propositions” at all. What are they? They are nothing but certain rules which determine the use of language, i. e. of expression by combination of “signs.” They are concerned with symbols, not with reality. They are nevertheless in a certain sense applicable to the world of facts, because symbols (words, letters, etc.) are used in speaking about facts. In short, they speak of reality, but they do not say anything about it. This can best be made clear by referring to simple examples. I shall first take a mathematical example, and afterwards a logical one.

You will all admit that there is absolutely no difference of meaning between the two sentences: “I have in my pocket twelve dollars” and “I have in my pocket seven plus five dollars.” Well, the famous “proposition” that seven plus five equals twelve is nothing but the rule which tells us that we may transform the one sentence into the other without changing the meaning.

You observe this is not the expression of any fact in the universe. It does not assert that there are twelve objects in the world, or seven or five; it does not assert that any one can count or has counted any objects: it just gives us to understand that a man who says “Here are twelve things” and another who says “Here are five plus seven things” have not said anything different, but have only used different words in order to express the same meaning (provided, of course, that they used the words five, seven and twelve in their ordinary sense; and when a little while ago I spoke of the great difficulties involved in this issue I had in mind chiefly the difficulty of clarifying the “ordinary sense”). That our arithmetical rule “applies” to any objects whatever is nothing remarkable or wonderful, for it does not say anything about any objects.

This point comes out perhaps even with greater clarity when we examine a purely logical example, as for example, the Principle of the Excluded Middle. When I say: “My friend will either come or not come tomorrow,” the logical principle just referred to assures me that this statement is always true, whenever and wherever it is made — but is that statement really a proposition? does it assert anything about my friend or his coming, or indeed about any other fact in the world? Evidently not. It speaks of my friend and his coming, but does not say anything about them, it asserts nothing whatever. After I have heard the sentence I know absolutely no more about the world than I did before; the sentence has communicated no fact to me. Now, the rules of logic and of pure mathematics represent all the truths which have their origin in “pure reason,” and of which it is so proud. But we see, that their “truth” and general applicability, which cannot be denied, are of a very particular, insignificant kind: they are merely formal, they do not convey any knowledge, they do not deal with any facts, but only with the symbols by means of which facts are expressed.

In this way the only apparent support of rationalism breaks down. The real nature of logical and mathematical “propositions” has been elucidated: the new empiricism does not deny their absolute truth and their purely rational character, but it maintains that they are “empty,” they do not contain any “knowledge” in the same sense as do empirical propositions; pure reason is unable to produce any real knowledge, its only business is the arrangement of the symbols which are used for the expression of knowledge.

After so-called “rational knowledge” is accounted for, empiricism now has the right and the power to claim the whole field of knowledge. We know nothing except by experience, and experience is the only criterion of the truth or falsity of any real proposition. You will remember that verifiability by experience was also the criterion of those questions that can in principle be answered. By holding these two results together we conclude that there are no really insoluble problems at all. All proper questions and all proper propositions (which can always be considered as answers to certain questions) are related to experience in the same way: they arise from it, and they can, in principle, be answered and tested by it. But what about those apparently insoluble problems which we seemed to discover among the questions of philosophy? “How do physical processes in the brain produce mental processes?” “Are animals or plants conscious beings or not?” What becomes of these problems?

Fortunately the same analytical methods which helped us to understand logic and mathematics (and at which I could not even hint in this lecture) permit us to answer the question and do away with all difficulties which it seems to hide. Again I can only indicate the result without being able to prove my point on this occasion. Reduced to the shortest formula the result is simply this: the so-called insoluble problems are in principle insoluble, because they are no problems at all. It is true they have the io grammatical form of questions, and in many cases it is very hard to distinguish them from real ones, but a close scrutiny of the words which occur in the question, and of the way in which they are combined shows that they violate the rules of logical grammar and therefore make no sense at all. I have tried to make this clear in my first lecture already, and we now see the importance of our former result from a different angle.

In the examples given above the analysis would have to elucidate the proper meaning in which the words “physical process” and “mental process” are supposed to be used in the first problem; and in the second problem the investigation would have to turn around the meaning of the word “consciousness;” and in both cases there would be two possibilities: either some meaning could be found for those words and their particular combination, so that the question would make perfectly good sense; in this case it would immediately be seen that the question would assume a harmless nature, that it would lose its great “philosophical” interest and would become an ordinary scientific problem, which would in principle surely be solved by the methods of observation and experiment. Or it will turn out that no meaningful interpretation of the words and their combination can be discovered; in this case the question has disappeared and what is left is nothing but a series of words put together in some confusing way by some confused mind.

Metaphysics disappears, not because metaphysical problems were insoluble, as most of the old empiristic schools believed, but because there are no such problems. Where there are no questions, there can be no answers; it would be absurd to look for a solution where there is no problem. Most of the older empiricists, it is true, denied the possibility of metaphysics with the same emphasis as we do, but, as I said before, not quite without a feeling of regret, because in their minds this meant a limitation of human knowledge, and consequently their attitude could justly be called skeptical (I may mention the names of Sextus Empiricus and of David Hume here).

But our new empiricism is in no way skeptical; on the contrary, it denies that there is in principle any limit to human knowledge; the existing limits are not of an essential, philosophical nature, but only accidental ones. They are only practical, technical, and may some time be overcome. According to our view all knowledge is based on experience, but this does no longer mean any restriction of knowledge. In the older views the impossibility of metaphysics was due to some regrettable imperfection or incapacity of the human mind; in our view the impossibility is of a logical order, it is due to some intrinsic non-sense in the phrases which were supposed to express “metaphysical problems.” To regret the impossibility of metaphysics becomes impossible; it would be the same as regretting the impossibility of a round square.

All real questions (i. e. those combinations of words to which we can possibly give the meaning of a question) can in principle be answered (“There is no ‘Riddle of the Universe’,” as Ludwig Wittgenstein has put it), and they can be answered by experience only, by the methods of science. A heavy burden is taken from Philosophy, and it cannot quarrel any more with science. Its function is analytic and critical, it helps us to get rid of mere verbal disputes, and with unspeakable relief do we see great “problems” vanish without leaving empty places.

The greatest difference between the older empiricism and our new philosophy of experience lies, I think, in its method. The former started with an analysis of human faculties (such as thinking, perceiving, and so forth); the latter starts with something much more fundamental, namely: the analysis of “expression” in general. All propositions, all languages, all systems of symbols, also all philosophies, want to express something. They can do this only if there is something there that can be expressed: it is the material of all knowledge, and to say that it must be given by experience is but another way of saying that something must be there before we can have knowledge of it and about it.

The position of this philosophy is unassailable, because it rests on the acknowledgment of the hardest facts and the study of the strictest logic. On these foundations our Philosophy of Experience stands very securely as on a firm rock amidst a wild sea of various philosophical opinions. It is neither skeptical nor dogmatic, it cannot interfere with science or with human values; its only object is understanding. Only in this way can it attain that serene attitude which belongs to all genuine philosophy: the attitude of Wisdom.