It is natural that mankind should take great pride in the steady advance of its knowledge. The joy we feel in the contemplation of scientific progress is fully justified. One problem after another is solved by science; and the success of the past gives us ample reason for our hope that this process will go on, perhaps even at a quicker pace. But will it, can it, go on indefinitely? It seems a little ridiculous to suppose a day might come when all imaginable problems would be solved, so that there would be no questions left for which the human mind would crave an answer. We feel sure that our curiosity will never be completely satisfied and that the progress of knowledge will not come to a stop when it has reached its last goal.

It is commonly assumed that there are other imperative reasons why scientific advance cannot go on forever. Most people believe in the existence of barriers that cannot be scaled by human reason and by human experience. The final and perhaps the most important truths are thought to be permanently hidden from our eyes; the key to the Riddle of the Universe is believed to be buried in depths the access to which is barred to all mortals by the very nature of the Universe. According to this common belief, there are many questions which we can formulate, and whose meaning we can grasp completely, though it is definitely impossible to know their answer which is beyond the nature and necessary boundary of all knowledge. In regard to these questions a final ignorabimus is pronounced. Nature, it is said, does not wish her deepest secrets to be revealed; God has set a limit of knowledge which shall not be passed by his creatures, and beyond which faith must take the place of curiosity.

It is easy to understand how such a view originated, but it is not so clear why it should be considered to be a particularly pious or reverent attitude. Why should Nature seem more wonderful to us if she cannot be known completely? Surely she does not wish to conceal anything on purpose, for she has no secrets, nothing to be ashamed of. On the contrary, the more we know of the world the more we shall marvel at it; and if we should know its ultimate principles and its most general laws, our feeling of wonder and reverence would pass all bounds. Nothing is gained by picturing God as jealously hiding from his creatures the innermost structure of his creation, indeed, a worthier conception of a Supreme Being should imply that no ultimate boundary should be set to the knowledge of beings to whom an infinite desire of knowledge has been given. The existence of an absolute ignorabimus would form an exceedingly vexing problem to a philosophical mind. It would be a great step forward in philosophy, if the burden of this bewildering problem could be thrown off.

This, one may argue, is evidently impossible, for without doubt there are unanswerable questions. It is very easy to ask questions the answers to which, we have the strongest reasons to believe, will never be known to any Human being. What did Plato do at eight o'clock in the morning of his fiftieth birthday? How much did Homer weigh when he wrote the first line of the Iliad? Is there a piece of silver to be found on the other side of the moon, three inches long and shaped like a fish? Obviously, men will never know the answers to these questions, however hard they may try. But at the same time, we know that they would never try very hard. These problems, they will say, are of no importance, no philosopher would worry about them, and no historian or naturalist would care whether he knew the answers or not.

Here, then, we have certain questions whose insolubility does not trouble the philosopher; and evidently there are reasons why it need not trouble him. This is important. We must be content to have insoluble questions. But what if all of them could be shown to be of such a kind as not to cause any really serious concern to the philosopher? In that case he would be relieved. Although there would be many things he could not know, the real burden of the ignorabimus would be lifted from his shoulders. At first sight there seems to be little hope for this as some of the most important issues of philosophy are generally held to belong to the class of insoluble problems. Let us consider this point carefully.

What do we mean when we call a question important? When do we hold it to be of interest to the philosopher? Broadly speaking, when it is a question of principle; one that refers to a general feature of the world, not a detail; one that concerns the structure of the world, a valid law, not a single unique fact. This distinction may be described as the difference between the real nature of the Universe and the accidental form in which this nature manifests itself.

Correspondingly, the reasons why a given problem is insoluble may be of two entirely different kinds. In the first place, the impossibility of answering a given question may be an impossibility in principle or, as we shall call it, a logical impossibility. In the second place, it may be due to accidental circumstances which do not affect the general laws, and in this case we shall speak of an empirical impossibility.

In the simple instances given above, it is clear that the impossibility of answering these questions is of the empirical kind. It is merely a matter of chance that neither Plato nor any of his friends took exact notes of his doings on his fiftieth birthday (or that such notes were lost if any were taken); and a similar remark applies to the questions concerning the weight of Homer and things on the other side of the moon. It is practically or technically impossible for humans to reach the moon and go around it, and such an exploration of our earth's satellite will never take place. But we cannot declare it impossible in principle. The moon happens to be very far off; it happens to turn always the same side towards the earth; it happens to possess no atmosphere which human beings could breathe - but we can very easily imagine all of these circumstances to be different. We are prevented from visiting the moon only by brute facts, by an unfortunate state of affairs, not by any principle by which certain things were deliberately held from our knowledge. Even if the impossibility of solving a certain question is due to a Law of Nature, we shall have to say that it is only empirical, not logical, provided we can indicate how the law would have to be changed in order to make the question answerable. After all, the existence of any Law of Nature must be considered as an empirical fact which might just as well be different. The scientist's whole interest is concentrated on the particular Laws of Nature; but the philosopher's general point of view must be independent of the validity of any particular one of them.

It is one of the most important contentions of the Philosophy I am advocating that there are many questions which it is empirically impossible to answer, - but not a single real question for which it would be logically impossible to find a solution. Since only the latter kind of impossibility would have that hopeless and fatal character which is implied by the ignorabimus and which could cause philosophers to speak of a "Riddle of the Universe" and to despair of such problems as the "cognition of things in themselves," and similar ones, it would seem that the acceptance of my opinion would bring the greatest relief to all those who have been unduly concerned about the essential incompetence of human knowledge in regard to the greatest issues. Nobody can reasonably complain about the empirical impossibility of knowing everything, for that would be equivalent to complaining that we cannot live at all times and be in all places simultaneously. Nobody wants to know all the facts, and it is not important to know them: the really essential principles of the universe reveal themselves at any time and any place. I do not suggest, of course, that they lie open at first glance, but they can always be discovered by the careful and penetrating methods of science.

How can I prove my point? What assures us that the impossibility of answering questions never belongs to the question as such, is never a matter of principle, but is always due to accidental empirical circumstances, which may some day change? There is no room here for a real proof;¹ but I can indicate in general how the result is obtained.

It is done by an analysis of the meaning of our questions. Evidently philosophical issues - and very often other problems too - are difficult to understand: we have to ask for an explanation of what is meant by them. How is such an explanation given? How do we indicate the meaning of a question?

A conscientious examination shows that all the various ways of explaining what is actually meant by a question are, ultimately, nothing but various descriptions of ways in which the answer to the question must be found. Every explanation or indication of the meaning of a question consists, in some way or other, of prescriptions for finding its answer. This principle has proved to be of fundamental importance for the method of science. For example, it led Einstein, as he himself admits, to the discovery of the Theory of Relativity. It may be empirically impossible to follow those prescriptions (like travelling around the moon), but it cannot be logically impossible. For what is logically impossible cannot even be described, i.e., it cannot be expressed by words or other means of communication.

The truth of this last statement is shown by an analysis of "description" and "expression" into which we cannot enter here. But taking it for granted, we see that no real question is in principle - i.e. logically - unanswerable. For the logical impossibility of solving a problem is equivalent to the impossibility of describing a method of finding its solution and this, as we have stated, is equivalent to the impossibility of indicating the meaning of the problem. Thus a question which is unanswerable in principle can have no meaning; it can be no question at all: it is nothing but a nonsensical series of words with a question mark after them. As it is logically impossible to give an answer where there is no question, this cannot be a cause of wonder, dissatisfaction, or despair.

This conclusion can be made clearer by considering one or two examples. Our question as to the weight of Homer has meaning, of course, because we can easily describe methods of weighing human bodies (even poets); in other words, the notion of weight is accurately defined. Probably Homer was never weighed, and it is empirically impossible to do it now, because his body no longer exists; but these accidental facts do not alter the sense of the question. Or take the problem of survival after death. It is a meaningful question, because we can indicate ways in which it could be solved. One method of ascertaining one' s own survival would simply consist in dying. It would also be possible to describe certain observations of scientific character that would lead us to accept a definite answer. That such observations could not be made thus far is an empirical fact which cannot entail a definite ignorabimus in regard to the problem.

Now consider the question: "What is the nature of time?" What does it mean? What do the words "the nature of" stand for? The scientist might, perhaps, invent some kind of explanation, he might suggest some statements which he would regard as possible answers to the question; but his explanation could be nothing but the description of a method of discovering which of the suggested answers is the true one. In other words, by giving a meaning to the question he has at the same time. made it logically answerable, although he may not be able to make it empirically soluble. Without such an explanation, however, words "What is the nature of time?" are no question at all. If a philosopher confronts us with a series of words like this and neglects to explain the meaning, he cannot wonder if no answer is forthcoming. It is as if he had asked us: "How much does philosophy weigh?" in which case it is immediately seen that is not a question at all, but mere nonsense. Questions like "Can we know the Absolute?" and innumerable similar ones must be dealt with in the same way as the " problem" concerning the nature of Time.

All great philosophical issues that have been discussed since the time of Parmenides to our present day are of one of two kinds: we can either give them a definite meaning by careful and accurate explanation and definitions, and then we are sure that they are soluble in principle, although they may give scientist the greatest trouble and may even never solved on account of unfavourable empirical circumstances, or we fail to give them any meaning, and then they are no questions at all. Neither case need cause uneasiness for the philosopher. His greatest troubles arose from a failure to distinguish between the two.