The proponents of an idea, as a rule, explain its non-acceptance differently than do the critics and opponents. Being myself much involved, it seems to me that the resistance met in the 18th century by Giambattista Vico, the first true constructivist, and by Silvio Ceccato and Jean Piaget in the more recent past, is not so much due to inconsistencies or gaps in their argumentation, as to the justifiable suspicion that constructivism intends to undermine too large a part of the traditional view of the world. Indeed, one need not enter very far into constructivist thought to realize that it inevitably leads to the contention that man — and man alone — is responsible for his thinking, his knowledge and, therefore, also for what he does. Today, when behaviorists are still intent on pushing all responsibility into the environment, and sociobiologists are trying to place much of it into genes, a doctrine may well seem uncomfortable if it suggests that we have no one but ourselves to thank for the world in which we appear to be living. That is precisely what constructivism intends to say — but it says a good deal more. We build that world for the most part unawares, simply because we do not know how we do it. That ignorance is quite unnecessary. Radical constructivism maintains — not unlike Kant in his Critique — that the operations by means of which we assemble our experiential world can be explored, and that an awareness of this operating (which Ceccato in Italian so nicely called consapevolezza operativa)  can help us do it differently and, perhaps, better.
This introduction, I repeat, will be limited to the exposition of a few aspects. The first section deals with the relation between knowledge and that "absolute" reality that is supposed to be independent of all experience, and I shall try to show that our knowledge can never be interpreted as a picture or representation of that real world, but only as a key that unlocks possible paths for us (see Alcmaeon’s fragment).
The second section outlines the beginnings of skepticism and Kant’s insight that, because our ways of experiencing are what they are, we cannot possibly conceive of an unexperienced world; it then presents some aspects of Vico’s constructivist thought. The third section explicates some of the main traits of the constructivist analysis of concepts. Some of the many ideas I have taken over from Piaget as well as from Ceccato will be outlined and only sparsely supported by quotations. Piaget’s work has greatly influenced and encouraged me during the 1970s, and before that, the collaboration with Ceccato had provided direction and innumerable insights to my thinking. But for constructivists, all communication and all understanding are a matter of interpretive construction on the part of the experiencing subject and, therefore, in the last analysis, I alone can take the responsibility for what is being said on these pages.
The history of philosophy is a tangle of isms. Idealism, rationalism, nominalism, realism, skepticism, and dozens more have battled with one another more or less vigorously and continuously during the 25 centuries since the first written evidence of Western thought.
The many schools, directions, and movements are often difficult to distinguish. In one respect, however, any ism that wants to be taken seriously must set itself apart from all that are already established: it must come up with at least one new turn in the theory of knowledge. Often that is no more than a rearrangement of well-known building blocks, a slight shift in the starting-point, or the splitting of a traditional concept. The epistemological problem — how we acquire knowledge of reality, and how reliable and "true" that knowledge might be — occupies contemporary philosophy no less than it occupied Plato. The ways and means of the search for solutions have, of course, become more varied and complicated, but the basic question has, almost without exception, remained the same. The way that question was put at the very beginning made it impossible to answer, and the attempts that have since been made could not get anywhere near a solution to the problem.
The philosopher of science, Hilary Putnam, has recently formulated it like this: "It is impossible to find a philosopher before Kant (and after the pre-Socratics) who was not a metaphysical realist, at least about what he took to be basic or unreducible assertions."
 Putnam explains that statement by saying that, during those 2,000 years, philosophers certainly disagreed in their views about what really exists, but their conception of truth was always the same, in that it was tied to the notion of objective validity.
A metaphysical realist, thus, is one who insists that we may call something "true" only if it corresponds to an independent, "objective" reality.  On the whole, even after Kant, the situation did not change. There were some who tried to take his Critique of Pure Reason seriously, but the pressure of philosophical tradition was overwhelming.
In spite of Kant’s thesis that our mind does not derive laws from nature, but imposes them on it,  most scientists today still consider themselves "discoverers" who unveil nature’s secrets and slowly but steadily expand the range of human knowledge; and countless philosophers have dedicated themselves to the task of ascribing to that laboriously acquired knowledge the unquestionable certainty which the rest of the world expects of genuine truth. Now as ever, there reigns the conviction that knowledge is knowledge only if it reflects the world as it is. 
The history of Western epistemology can, of course, not be described adequately and fairly in a few pages. Given the limits of this chapter, it will have to suffice if I pick out the main point in which the constructivism I am proposing differs radically from the traditional conceptualizations. That radical difference concerns the relation of knowledge and reality. Whereas in the traditional view of epistemology, as well as of cognitive psychology, that relation is always seen as a more or less picture-like (iconic) correspondence or match, radical constructivism sees it as an adaptation in the functional sense.
In everyday English, that conceptual opposition can be brought out quite clearly by pitting the words "match" and "fit" against one another in certain contexts. The metaphysical realist looks for knowledge that matches reality in the same sense as you might look for paint to match the color that is already on the wall that you have to repair. In the epistemologist’s case it is, of course, not color that concerns him, but he is, nevertheless, concerned with some kind of "homomorphism," which is to say, an equivalence of relations, sequence, or characteristic structure — something, in other words, that he can consider the same, because only then could he say that his knowledge is of the world.
If, on the other hand, we say that something fits, we have in mind a different relation. A key fits if it opens the lock. The fit describes a capacity of the key, not of the lock. Thanks to professional burglars, we know only too well that there are many keys that are shaped quite differently from ours but nevertheless unlock our doors. The metaphor is crude, but it serves quite well to bring into relief the difference I want to explicate. From the radical constructivist point of view, all of us — scientists, philosophers, laymen, school children, animals, indeed any kind of living organism — face our environment as the burglar faces a lock that he has to unlock in order to get at the loot. This is the sense in which the word "fit" applies to Darwin’s and neo-Darwinist theories of evolution. Unfortunately, Darwin himself used the expression "survival of the fittest."
In doing that, he prepared the way or the misguided notion that, on the basis of his theory, one could consider certain things fitter than fit, and that among those there could even be a fittest.  But in a theory in which survival is the only criterion for the selection of species, there are only two possibilities: either a species fits its environment (including the other species), or it does not; i.e., it either survives, or it dies out.
Only an external observer who introduces other criteria — e.g., economy, simplicity, or elegance of the method of surviving — only an observer who deliberately posits values beyond survival, could venture comparative judgments about those items that have already manifested their fitness by surviving. In this one respect the basic principle of radical constructivist epistemology coincides with that of the theory of evolution: Just as the environment places constraints on the living organism (biological structures) and eliminates all variants that in some way transgress the limits within which they are possible or "viable," so the experiential world, be it that of everyday life or of the laboratory, constitutes the testing ground for our ideas (cognitive structures). That applies to the very first regularities the infant establishes in its barely differentiated experience, it applies to the rules with whose help adults try to manage their common sense world, and it applies to the hypotheses, the theories, and the so-called "natural laws" that scientists formulate in their endeavor to glean lasting stability and order from the widest possible range of experiences.
In the light of further experience, regularities, rules of thumb, and theories either prove themselves reliable or they do not (unless we introduce the concept of probability — in which case we are explicitly relinquishing the condition that knowledge must be certain).
In the history of knowledge, as in the theory of evolution, people have spoken of "adaptation" and, in doing so, a colossal misunderstanding was generated. If we take seriously the evolutionary way of thinking, it could never be organisms or ideas that adapt to reality, but it is always reality which, by limiting what is possible, inexorably annihilates what is not fit to live. In phylogenesis, as in the history of ideas, "natural selection" does not in any positive sense select the fittest, the sturdiest, the best, or the truest, but it functions negatively, in that it simply lets go under whatever does not pass the test.
The comparison is, of course, stretched a little too far. In nature, a lack of fitness is invariably fatal; philosophers, however, rarely die of their inadequate ideas. In the history of ideas it is not a question of survival, but of "Truth." If we keep that in mind, the theory of evolution can serve as a powerful analogy: the relation between viable biological structures and their environment is, indeed, the same as the relation between viable cognitive structures and the experiential world of the thinking subject. Both fit — the first because natural accident has shaped them that way, the second because human intention has formed them to attain the ends they happen to attain; ends that are the explanation, prediction, or control of specific experiences.
More important still is the epistemological aspect of the analogy. In spite of the often misleading assertions of ethologists, the structure of behavior of living organisms can never serve as a basis for conclusions concerning an "objective" world, i.e., a world as it might be prior to experience.  The reason for this, according to the theory of evolution, is that there is no causal link between that world and the survival capacity of biological structures or behaviors. As Gregory Bateson has stressed, Darwin’s theory is based on the principle of constraints, not on the principle of cause and effect.  The organisms that we find alive at any particular moment of evolutionary history, and their ways of behaving, are the result of cumulative accidental variations, and the influence of the environment was and is, under all circumstances, limited to the elimination of non-viable variants. Hence, the environment can, at best, be held responsible for extinction, but never for survival. That is to say, an observer of evolutionary history may, indeed, establish that everything that has died out must in some way have transgressed the range of the viable and that everything he finds surviving is, at least for the time being, viable. To assert that, however, evidently constitutes a tautology (what survives, lives) and throws no light whatever on the objective properties of that world that manifests itself in negative effects alone.
* Originally published in Die Erfundene Wirklichkeit, a volume edited by P. Watzlawick (Munich:Piper, 1981, pp. 16–38). English translation in The Invented Reality (New York: Norton, 1984,pp. 17–40).6 ANTIMATTERS 2 (3) 2008