As with many theories, it seems like there is a certain core of truth to this. It is true that our knowledge of the world (there are after all things we can know without reference to the world -- mathematical knowledge, for example) is filtered through our ability to correctly understand what we observe, which in turn is affected by our personal and cultural biases. Yet, the value of this mental construction of the world is the degree of similarity it bears to the real world which does indeed exist outside of us. Focusing on the importance of building one's own model while ignoring its degree of semblance to the real world makes the whole concept of knowledge irrelevant, as this article Fred links to points out:
Accordingly, most constructivists are relativists (although some disclaim it). For relativists there is no truth, only "truth"-truth in quotes-"a rhetorical pat on the back," as one noted relativist philosopher has explained-a compliment accorded that which is agreed to in some community by that community, but no more than that. Hence there is no robust connection between science and some universal, external reality. To a social constructivist,42 in particular, there can be no "knowledge"; there are only knowledges. Not only "Western" science, then, knows the physical world; science is no better a way of knowing it than many other, very different ways of knowing.All in all, it sounds like constructivism has some very useful ideas, but (especially when combined with our current culture's tendency towards relativism and self-esteem worship) can also lead in intellectually destructive directions.
That is the force of the statement that "science is a mental representation constructed by the individual." Outside the individual, in other words, there is no independent reality to which "knowledge" or "truth" corresponds. Knowledge of the world is in each human mind, where it is constructed from prior and current experience. Some constructivists insist that they are not anti-realists-they do not reject reality; only "objectivism" (often misidentified by philosophic amateurs with "positivism"), their label for claims that knowledge can be free of personal and cultural bias. For a serious constructivist, there is no knowledge free of cultural bias. To which, then, the last of these quotations is the appropriate conclusion: that the history of science is no more than "the changing commitments of scientists . . . . [which] forge changes commonly referred to as advances in science." Meaning: there is no real progress toward truth about the physical world. Over the centuries, there have been only changing opinions about it, reached by negotiation and power shifts among contending parties. Scientists just use the term "advances" to label any changes to which they are finally agreed.
Exploring this bizarre amalgam of postmodernism, epistemological relativism, and old learning theory, the astounded layman may well ask, "How can anyone teach natural science under a theory of science so hostile to its purposes, so blind to its practices and achievements?" The full answer is more than "Well . . . they don't teach it," although that is a part of it.