[This is an essay I wrote for some friends. We have a little group that gets together once in a while to discuss science books we've read.]

1. Introduction. Once again the question of whether we can ever be said to know anything for certain has come up in our science discussion group. Some notions never die; they can't die as long as we live in a milieu that continually fosters them. And the notion that "nothing can be known for certain" is certainly one of these.

It is ironic that I find I have to defend the view that we do know many things, and indeed that we know some things for certain—because I am probably the most skeptical person in the whole group! But as I will try to show below, there are also irrational forms of skepticism, such as "doubting everything". And actually there is a hidden unity in the thinking of many who claim (on the one hand) to "doubt everything to some degree", and yet who also strongly believe in many irrational things. The central problem is that such people do not have any scientific standards of belief and knowledge, and therefore tend to go wrong in both directions.

2. What is Knowledge? What is knowledge anyway? What is the definition of the term? Is the definition such that nothing can possibly be said to really be knowledge? (If so, why would we have such a term?)

Actually, knowledge is a fairly simple concept. Here is the complete definition from one of the better short dictionaries currently available (Merriam-Webster's Collegiate Dictionary, 10th ed.), omitting only obsolete and archaic meanings (such as "knowing someone" in the Biblical sense!):

Now none of that is very frightening or impossible to achieve. As the dictionary adds (in its discussion of various synonyms), knowledge is just the "facts or ideas acquired by study, investigation, observation, or experience." All (normal) human beings have acquired vast amounts of knowledge, and there is nothing really extremely demanding about doing so.

Philosophers, however, have tried to deepen this definition. They have generally agreed that knowledge is (approximately) "justified true belief". In other words, you can be said to know something if three conditions hold.

1. You believe it.
2. You are justified in believing it. (E.g., you have good evidence or reasons.)
3. It is actually true.

(I should note that there is an enormous amount of discussion about this definition in the philosophical literature. The reason is that this definition can be quibbled with ad infinitum. It is possible to cook up exotic examples where the three conditions hold and yet there may be some possible debate about whether the person can be truly said to know the thing, and other exceptional cases where you might possibly be said to know something even though all three conditions may not quite hold. But despite all this quibbling, I think it is fair to say that "justified true belief" is an excellent capsule definition of 'knowledge'. If it is not always precisely correct in every single case, it is very nearly so.)

But this philosopher's definition is funny in one way. Here is the way philosophers typically present the difficulty that now arises: It is generally easy to determine if someone believes something to be true, and usually it can also be easily determined if they have good reasons for believing it. But how do you go about demonstrating the third condition—determining for certain that something is actually true? It is important to do this because if it turns out that the belief is actually false we are then forced to say that the person did not really "know it" after all, regardless of how good their reasons were.

Ah, philosophers! Did you notice the intellectual sleight of hand that went on in the last paragraph? They start off talking about what it means to know something, but surreptitiously slip into talking about how you know for certain that somebody knows something. That's very different! I believe that one of the reasons this sort of thing happens is that 20th century Anglo-American philosophy has been obsessed with formal logic, and constantly tries to reformulate every problem in terms of it. They are not happy unless they can establish everything with "logical certainty". (This fixation derives partly from Russell & Whitehead's Principia Mathematica.)

Truth, in particular, is a concept that many philosophers seem to believe "belongs" to logic. Or they at least believe that truth must be discussed from the point of view of formal logic. (Actually the concept of truth is prior to any system of logic; i.e., that there are such things as true statements is a given, a prerequisite, for the construction of any system of formal logic.) 'Truth' is in truth no great mystery, despite the pseudo-profundity of all the Pontius Pilate-like bourgeois philosophers of the world. (As Paul Ziff put it: "Pontius Pilate asked 'What is truth?' but would not stay for an answer.") Truth is simply (as the dictionary says) "the state of being the case".

It is very often quite easy to determine if something is true. Is it true that the book is on the table? Just turn your head and see.

But I want to return to and reemphasize the point that philosophers unconsciously seek to force simple matters of knowing into stricter matters of knowing for certain. And not just philosophers, but also other people, and especially scientists and other intellectuals, who have been influenced (far more than they realize) by the prevailing philosophy. And knowing for certain is in turn always pushed toward the impossible standard of it being inconceivable that you could turn out to be wrong. No matter what the standard of knowledge, it is not good enough for some people. If there is even the remotest possibility that you might turn out to be wrong, then they want to say that you do not really know the thing for a fact, or at least that you do not know it for certain.

In short, such people will accept only the strictest conceivable standards for knowledge, and that is ridiculous. Actually we have a whole series of standards for knowledge, standards that become stricter by degree, and we have words and phrases to indicate which of these various standards is being applied. It is incorrect to misuse these words and phrases by demanding stricter standards than are appropriate to them.

3. Standards of Knowledge. To judge anything whatsoever you must have standards; standards appropriate to that particular sort of judgment. Thus at a dog show there are certain types of standards the judges use, and even there, the standards used to judge dachshunds are not necessarily those used to judge St. Bernards. Short legs, for example, are a virtue in dachshunds, but a negative characteristic in St. Bernards. Moreover, when you categorize a dog as a dachshund you are able to do so by (at least implicitly) referring to those very same standards. To categorize is to implicitly judge in light of certain standards.

We all categorize our various kinds of beliefs and knowledge too. Any time you categorize some assertion as a "fact", or as "something we know", or as "something that is certainly true", or as something which is "beyond all doubt", you are judging it according to a set of implicit standards of knowledge. And those standards vary somewhat according to the precise phrase used. Another way of putting it is to say that these qualifying phrases exist for no other reason than to indicate which sorts of standards are appropriate in the given case.

Let's look at some of the ways in which we categorize knowledge. Let X be any statement, such as: "The earth goes around the sun." or "No human beings are presently living on Mars."

1. X. (I.e., simply stating that "The earth goes around the sun.", etc.)
2. It is a fact that X.
3. We know that X.
4. We know for sure that X.
5. We know for certain that X.
6. We know, beyond any reasonable possibility of error, that X.
7. We know with scientific certainty that X.
8. It is absolutely certain that X.
9. We know as well as we know anything that X.
10. We know, beyond any possibility of error whatsoever, that X.
11. It is logically certain that X.
12. It is impossible to conceive that X is false. (Or, it is impossible to conceive that X might turn out to be false.)

Our most usual method of reporting something that is known is 1. Methods 2 and 3 are used when it is desired to emphasized that X is known, and is not just someone's belief.

The standards of knowledge implicit in the first three variations are more or less the same, and are just those which we use for scientific knowledge in general, as well as every-day knowledge (such as that my name is Scott, I live in California, etc.) These are tough standards, and there are many things that are probably true, things for which we have some good reasons for believing, but which are not known to be true. (That there is no life on Jupiter, for example, or that I will never win the lottery.) But the remaining variations imply standards which are even tougher, and which get progressively tougher as we go down the list.

The standard implicit at level 4 and 5, for example, says that this is not just run-of-the-mill, average knowledge we are talking about, but something for which we have exceptionally good, or overwhelming evidence. If knowledge is "justified true belief", then "certain knowledge" is exceptionally well justified true belief. It is a plain and simple fact that the evidence we have for some of the things we know is much greater than the evidence we have in other cases. We do know for certain that the earth goes around the sun, and to express it this way is just to emphasize that our reasons for believing this are indeed extremely or exceptionally good. To deny that we know for certain that the earth goes around the sun is thus to deny that our evidence for this is anything beyond the ordinary, and that would be incorrect. To say that we do not know for certain that the earth goes around the sun is to suggest—quite falsely—that there is some reasonable doubt about it.

The standard implicit at level 6 and 7 is a bit stricter yet, but when you start using the word 'absolute' (level 8) there is a qualitative jump. The standard implied at 8 and 9 is one of virtual impossibility of any error. Another way to look at it is: What else would we need to throw out if it turns out we are wrong about this belief? If it were to really turn out that the earth does not go around the sun, then we would have to throw out a very large part of what we know about astronomy and science in general. Once you get to the point where the alternative is throwing out just about everything science thinks it knows, it is more than justified to say that we know the thing with absolute certainty. Therefore I have no qualms at all in saying that it is absolutely certain that the earth goes around the sun. There really is no (scientific) doubt about it whatsoever.

At level 11 the standard explicitly indicated is that of formal logic. This is indeed a very high standard, but it is usually not applicable. It is not applicable to the case of the earth going around the sun for example, since there is no logical contradiction either way. However, we can know with logical certainty that the Christian God does not exist, since he is supposed to be all-knowing, all-powerful, and all-good, which logically precludes the existence of evil (which certainly does exist).

By the time you get to 12 the standard is so ridiculously high that nothing can possibly meet it (Descartes notwithstanding). A standard that cannot possibly be met is a pretty much useless standard, and that is why you seldom even hear it (except in philosophy classes!). But please note that the standard at 12 is not the same as the standard at 4 or 5. Lots of goofy things are conceivable! It is conceivable that I might really be just a brain in a vat somewhere, hooked up to a powerful computer that stimulates my nerves (optic, tactile, etc.) in such a way that I seem to see and feel things which are not in fact there. Perhaps in this scenario the earth and sun never even existed; my brain and vat might be in a different galaxy. But as I say, this absurd standard is almost never appropriate, and except in science fiction and philosophy class it should be totally rejected as a reasonable standard of knowledge.

In summary, we must not use or imply standards of knowledge that are ridiculously inappropriate.

4. Contemporary Science and Philosophy. Many people are unaware of how much science and philosophy influence each other. Scientists generally try to interpret their new discoveries and theories in terms of the prevailing philosophies. (In a bourgeois world, this almost always means in terms of bourgeois philosophies.) Philosophers, in turn, show great interest in major new scientific theories, especially physical theories, whether it be Newtonian physics, relativity theory, quantum mechanics, multi-dimensional string theory, or whatever, and build new philosophical world outlooks inspired in part by such theories. Quite often these scientists and philosophers are in fact the same group of people. Niels Bohr was more of a philosopher of science than he was a physicist (even though he was a much better physicist than he was a philosopher!)

Moreover, there is a very strong bandwagon effect in science and philosophy (as in intellectual circles in general). Over the past century many scientists, and especially physicists, have followed behind the faddish currents in bourgeois philosophy like little puppies. At the beginning of the 20^th century the dominant philosophical outlook among scientists was a kind of naive materialism, corrupted however by the frequent incompatible assumption that God exists, or may exist. With the advent of relativity theory and quantum mechanics, materialism of any kind (naive or otherwise) was thrown out, and various forms of idealism became practically mandatory in the scientific community. During the 1930s physicists flocked towards logical positivism and the Vienna Circle. This extreme form of empiricism was pretty crude however, and so they soon shifted into the variation of that school developed by Karl Popper. More recently even Popperian positivism has been losing ground to more blatant forms of idealism, such as the religious physics championed by Steven Hawking, Roger Penrose, Paul Davies, and the notorious Frank Tipler.

But despite the inroads of openly religious ideas into contemporary physics, the fundamental outlook characterizing the field is still basically that of Karl Popper. And one of the main tenets of that worldview is a doctrinaire philosophical agnosticism.

(One of Popper's central dogmas is that you can never prove any theory to be correct or know that it is true, but you can disprove theories by finding facts incompatible with them. This is itself a theory, and one of the many problems with it is that it applies to itself! That is, this doctrine of falsification is a theory that Popper puts forward as being true even though the theory itself says that you can never show any theory to be true. A more basic problem with it is that it fails to recognize that facts are only facts in terms of a theory. (Or, putting it another way, there is no hard line between a fact and a theory.) Therefore to adduce "facts" which falsify one theory is really to implicitly apply another theory; but how do you know that theory is true? Moreover, this falsification doctrine is unidimensional, neglecting many other characteristics a scientific theory must possess—such as internal coherence and compatibility with other (especially broader) theories. In short, Popper's falsification theory is quite wrong. Even most bourgeois philosophers have rejected it as simplistic at best, but in the sciences (especially physics) it is generally accepted as gospel. You may take it as a given: scientists are always bad philosophers.)

5. Philosophical Agnosticism. The term 'agnosticism' in ordinary speech usually means doubt about the existence of God, or (more strongly), the view that no one can know whether or not God exists. Philosophical (or epistemological) agnosticism is a generalization of that view to all of knowledge: it is the theory that no one can ever really know anything, or at least that no one can know anything for certain. Why? Because it is conceivable that you might turn out to be wrong! (That's right, the silly argument I already discussed and rejected earlier—rejecting it because it unreasonably demands an inappropriate and impossible standard for knowledge.)

Quite bizarrely, but under the influence of Popper, many people—including a great many scientists themselves—imagine that philosophical agnosticism is the fundamental assumption of modern science. Although everyone would agree that there is an enormous amount of evidence for some scientific beliefs, and that there is a lot more evidence for some beliefs than for others, they still want to say that you can never have enough evidence that you are justified in saying that you know the thing for sure. In fact many people are so much under the influence of this view that not only do they reject any claims that we know some things for certain, they are even quite uncomfortable with claims that we know anything period.

"We live in an age of uncertainty..." How many times have you heard that?! And yet more is known now than ever before. Scientific knowledge is increasing at a very rapid pace. So why is it that people lean so strongly towards agnosticism in the present age, of all times?! As I already mentioned, in part this is due to 20^th century developments in physics that have forced people to admit that some important things they thought they knew about the world are not in fact true. But in other spheres too, the old certainties of the 19^th century have faded away. The comfortable expectations of permanent peace, prosperity and steady social progress were shattered by world wars, the Great Depression and "frightening" revolutions. In the arts all sorts of "crazy" new forms have appeared, from atonal music to nonobjective painting. Traditional forms of religion have been seriously questioned and challenged, and new religions have entered the market place. With the advent of atomic weapons even the continuation of the human species can no longer be taken for granted. With all the old verities smashed, it is no wonder that bourgeois society entered a period of great self-doubt, and that agnosticism has become the dogma of the age.

But really this should all be considered rather amusing! It is like six-year-olds learning that Santa Claus doesn't exist and then refusing to believe anything at all their parents tell them. I know one little kid like this, Robin, the nephew of a friend of mine. One evening recently he asked me "Are motor cycles really dangerous?" He had been told by his parents that they are, but he didn't want to believe it, so he was checking with other "authorities"! But scientists have no authorities to turn to except the physical (and social) world itself, and must therefore stew perpetually in their own uncertainty if they are under the spell of Popper.

We sometimes do discover that something we thought we knew is wrong. But that does not mean we "really" know nothing, or that everything else we think we know might turn out to be wrong! When we discover that we are wrong about some such thing it should indeed make us a bit more skeptical, but primarily about beliefs of the same sort. That is, beliefs that are supported to the same degree. We need not become skeptical about beliefs that are supported to a much great degree, let alone beliefs that are supported by absolutely conclusive evidence.

Suppose you have an acquaintance who lies to you sometimes. When you discover this it is wise and correct to become more skeptical of what that person says in general. However, it would be unjustified to become more skeptical of everybody, since you already know that there are a few people out there who tend to lie a lot, and it is enough to recognize that this acquaintance is one of them. The increase in skepticism must be appropriate to the discovery of error on your part.

6. Scientific Skepticism. Let's look at a couple of Richard Feynman's comments on the topic of scientific certainty:

Is Feynman correct? Is science really completely uncertain? Is it all just a bunch of elaborate "guesses"? No, it is not. You can see why Feynman might want to say such things to people who know little of science and who imagine that science consists of a huge body of facts, all (or most) known with certainty. That is definitely not the case. But though it may be appropriate from a pedagogical point of view to emphasize the uncertainty in science, and even to suggest it is nothing but an elaborate system of "guesses", that is really quite silly if taken literally. To guess, says the dictionary, is "to form an opinion of from little or no evidence". But of course the point of science is to develop theories that are based on good evidence, and in the best cases, on overwhelming evidence. These theories in turn allow us to make scientific predictions which are not mere guesses, but for which we have good reasons, and in some cases the best possible reasons, i.e., complete certainty. When you predict that if you let go of your apple it will fall to the floor, that is not a guess, but a sure thing (provided of course there are no hidden strings, ferocious up-draughts, special electric fields, etc.).

It is incorrect to say either that "science is certain" or that "science is uncertain". There is both certainty in science and uncertainty—just as there is in everyday knowledge. Most of the accepted facts in science do have some small degree of uncertainty associated with them, as do most theories. But there are some facts and theories in science that are known with complete certainty. (There is no doubt or guesswork about them whatsoever.) No total or absolute skepticism is justified either in science or in everyday life, and to suggest otherwise is just an error on Feynman's part.

A few of the many scientific facts and theories that are now known with complete certainty (to science, though not to every human being) are:

• The earth orbits the sun.
• The moon orbits the earth.
• The earth is (more or less) a sphere.
• The earth spins on its axis (relative to the stars).
• The earth was formed vastly longer ago than 4,004 BC.
• Water is composed of two atoms of hydrogen and one of oxygen.
• Table salt is sodium chloride (plus impurities).
• Plant and animal species on earth have evolved from earlier, simpler organisms.
• Human beings require oxygen to live.
• (Ordinary) heat is at bottom the motion of molecules.
• Some atoms are radioactive.
• Light is a form of electromagnetic radiation.
• The human organ responsible for intelligence and other mental capabilities is the brain.
• Contemporary society consists of various social classes.

We could go on and on. At one time none of these things was known at all. Furthermore, each was at one time only guess-work, only a hypothesis. Not only that, but there are undoubtedly still more that can be learned about each of these things, and the way we express these truths may even change somewhat in light of some deeper understanding. But these facts (and many others) are nevertheless now known with certainty, and science will never change its mind about these basic truths.

When Einstein put forward relativity theory we learned some new details about the precise way that the earth orbits the sun. When deuterium was discovered, our concept of hydrogen changed somewhat. When the possibility of ionization was discovered, we somewhat changed our conception of what it means to say that molecules (such as water) are composed of atoms. But even before these discoveries we still knew that the earth goes around the sun, and that water is composed of hydrogen and oxygen. Even if there are an endless series of such refinements of our knowledge (as I believe there will be), it is still quite true that we know some things for certain right now.

We will find out some more details about these basic truths, what caused them, and so forth, and perhaps the solar system will explode someday and some of them will no longer be true. But as for now, they are scientific facts, known beyond any rational doubt whatsoever to be true—definitely and certainly true.

Many other things are known with equal certainty: that human beings (as we are presently constituted) generally have two arms and two legs (I'm tempted to say that's as plain as the nose on your face); that human muscle power is insufficient for any human to jump from the earth to the moon unaided; that lightning is not caused by Zeus hurling thunderbolts! Science will never change its mind on these points! Anybody who denies that we know these things for certain has been corrupted by a screwy epistemological theory to the point where they cannot think straight on the issue any more. No total skepticism about human knowledge is ever justified.

I don't wish to condemn skepticism altogether; it is an extremely important component of the scientific outlook. In general we should be skeptical to various degrees about most scientific theories. For many of them only a small degree of skepticism and doubt is justified, and usually "just around the edges". That is, our occasional skepticism about well-established and well-tested theories should mostly be focused on issues such as wondering if the scope of the theory is really quite as broad as has been claimed. Relativity theory, for instance, requires some degree of skepticism, but especially under extreme conditions such as in the sub-atomic world, or inside black holes where it clearly breaks down.

It may even be fair to say that all broad scientific theories require some degree of skepticism—just because they are so broad. Many of the details of the theory of evolution, for instance, are still in question, and some may always be. (But that doesn't mean there is any scientific doubt about the basic fact of evolution. The basic theory of evolution is established beyond any reasonable doubt; i.e., with scientific certainty.) The same is true of Marxism, incidentally. There are many things about it which are still subject to reasonable doubt, dispute, and possible amendment. But there is a core of basic ideas (such as the existence of the class struggle, and the need for revolution) which have been established beyond any reasonable scientific doubt. (The fact that many people do not understand this or believe it, is irrelevant to science, just as with the case of the many creationists who reject evolution.)

Some theories, especially the newer, less well-developed, less well-considered, and most importantly of all, less well-tested ones, require much more skepticism. You might believe the theory, and even have considerable evidence, but you should still be somewhat skeptical. Take the Alvarezes' theory that the cretaceous extinction 65 million years ago was caused by the impact of a comet or asteroid. I believe that theory is correct, and there is more and more evidence for it. But I wouldn't say that it is conclusively proven. It could still reasonably turn out to be wrong. So some skepticism—albeit a steadily decreasing amount—is still quite justified in regard to this theory.

You don't need to be a philosophical agnostic in order to be appropriately skeptical. It is interesting that a person such as myself, who argues that we do know many things with scientific certainty, tends to be far more skeptical about many contemporary scientific theories than do those who champion philosophical agnosticism. Take the Big Bang, for instance. Virtually no cosmologist today has any real doubt about that theory. There is some important evidence for it (e.g., the 2.7° cosmic background radiation, galactic red-shifts, the ratio of hydrogen and helium to each other and to other elements, etc.). Such facts (or presumed facts) do strongly suggest an expanding universe (or region of the universe), which may be the result of a colossal explosion, the "Big Bang". But there are other explanations possible here, and the Big Bang definitely deserves a great deal of skepticism—even with respect to this basic core of it (let alone the more absurd extrapolations to 10 to the -43 seconds!). And some parts of the Big Bang theory as usually presented (by Hawking for instance) can and should be rejected as idealist nonsense, such as the notion that the universe was "created" in the Big Bang, and that neither time, nor space, nor matter, nor anything else existed before it—except God, "of course"!

Many physicists who champion epistemological agnosticism nevertheless believe quite strongly in the wildest sorts of theories. Richard Feynman for example seemed to seriously believe that there might be only one electron in the entire universe, which goes back and forth in time creating the illusion of countless trillions of electrons. Murray Gell-Mann strongly upholds the nonsensical "many-worlds" interpretation of quantum mechanics. David Mermin, a physicist at Cornell University, claimed that because of quantum mechanics (the Copenhagen Interpretation this time) "We now know that the moon is demonstrably not there when nobody looks."

What are we to think about people who claim to "know" such laughable nonsense while at the same time denying that we can know "for certain" such proven facts as that the earth goes around the sun? It seems to me that in order to smooth the path for ridiculous beliefs they need to first try to destroy people's beliefs (including their own) in things that really are known beyond any reasonable doubt.

For my part, I am very suspicious of people who say that "really" we don't know anything. Just what agenda do they have anyway? Just what alternative beliefs are they putting forward, and why?! Here is Harrison's 73rd law: People who say that everything you know is really questionable are actually trying to indoctrinate you with some new system of beliefs which you are supposed to accept unquestioningly. Some may jump on this to say that it applies to Marxists too. But Marxists have always embraced the sciences and the general body of scientific knowledge. We believe we are adding to that body by developing the science of society—Marxism. And while some individual Marxists—like any other group of people—are dogmatic about their beliefs, the main creators of Marxism (e.g., Marx, Engels, Lenin & Mao) opposed dogmatism and favored a scientific approach.

7. The Dialectical Theory of Knowledge. One of the other leading lights in contemporary bourgeois philosophy of science is Thomas Kuhn, who just died recently. His book, The Structure of Scientific Revolutions, is like the second volume of gospel (along with Popper) among scientists. One of his main arguments is that at any time we may undergo a new revolution in any branch of science which will necessitate throwing out everything we thought we knew in that area, and starting over entirely from scratch.

It just ain't so. We virtually never really go back to square one. Instead, new knowledge, including radically new theories which reorganize such knowledge, expands on and extends existing knowledge. Usually only a little bit of what we thought we knew needs to be thrown out (though they may be some key theoretical points). The picture painted by Einstein & Infeld in their classic, The Evolution of Physics (1938), is much superior to Kuhn's view:

To extend the metaphor: The progress of science consists in climbing an endless series of hills, each allowing you to see a bit further, and to connect up a wider range of phenomena. At each new peak, there are new truths which become apparent, and some old ones which need to be reinterpreted in light of the better theory. And some older ideas, which we formerly thought to be true, will now be recognized as false, or at least as seriously misleading. But despite that, there will also remain truths which are basically unaffected by the new theory, and which will never be overthrown. Thus relativity theory did not overthrow everything we thought we knew (such as that the earth goes around the sun); nor will the future corrections to relativity theory that must inevitably come along! No matter how much smarter we get, no matter how high the hill we climb, the earth still goes 'round the sun; that is a certainty!

The way we Marxists put it is this: human knowledge develops in spirals, with leaps from time to time (scientific revolutions), but never throwing out everything and starting over from scratch.

Marxism is totally opposed to philosophical agnosticism—not because Marxism is "dogmatic", but because it is scientific. Way back in 1845, in the second of his marvelously profound "Theses on Feuerbach", Marx wrote that

In short, the reason we know that our ideas about the world are correct is that our practice, our practical activity, shows we can change the world by applying these ideas. We know they are correct to the extent they actually work in practice.

In his Dialectics of Nature, Engels also discusses this issue of whether or not the world is knowable to human beings. He criticizes at length Kant and his theory that the essence of things (the ding-an-sich, or the "thing in itself") is unknowable. The non-Marxist Leszek Kolakowski sums up Engels' views on this fairly well:

It is true that Engels upheld the "relativity of knowledge", but by this he meant such things as [See Kolakowski, pp. 395-6.]:

1. Our knowledge is always incomplete.
2. New scientific laws (explanations, theories) often replace older ones. These new laws do not usually contradict the older ones, but merely narrow the scope where the old laws apply. However, we can never be completely sure that we have narrowed down the scope of the law to its "final" limits of application, or that it will never be possible to reformulate the law more precisely in the future.

Here, as usual, we see that Marxist philosophy from a hundred years ago or more is much more sophisticated and correct than the latest in bourgeois philosophy of science. Scientists could learn a lot from Marx, Engels and other Marxists, but of course they are afraid to even look because of their ideological blinders.

8. Conclusion. In saying that many things are known for certain, or that some things are known with scientific certainty, I am of course not denying that many people are very apt to be unaware of those facts. Just because it is true that there is no longer any reasonable scientific doubt about some point or other in no way prevents the scientifically ignorant from having all kinds of doubts!

There are in fact today many "belief systems" which are incompatible with some or all of science. Traditional religions and "New Age" beliefs are cases in point. Yet another is the so-called "post-modernism" viewpoint which is very influential at universities these days, and which has adopted philosophical agnosticism as one of its most central and unquestioned tenets.

Take as an example just one of these worldviews, that of the fundamentalist Christians. They say the earth was created circa 4,004 BC; but science knows for certain that the earth is very ancient (best current estimate being about 4.6 billion years old). If people are not educated in a science, or reject its findings for some reason, then they will not know what those versed in that science know—not even those things that the science has established for certain. There are many facts in Marxist theory too, such as the existence of the class struggle as the central motive force in the history of human society, that have been established beyond any rational doubt, but which most people are nevertheless either ignorant of or reject for ideological reasons. But that only goes to show that we live in a very unscientific world.

And what about my favorite example of something that we know for certain, that the earth goes around the sun? "In a random survey of more than 2,000 adults, conducted by the Public Opinion Laboratory at Northern Illinois University, 21 per cent of the respondents said they believed that the sun revolved around the earth; an additional 7 per cent said they did not know which revolved around which." [Diane F. Halpern, "The War of the Worlds: When Students' Conceptual Understanding Clashes With Their Professors'", The Chronicle of Higher Education, March 12, 1997, p. B5.]

As I love to say, we live in primitive times.