"The common-sense theory of knowledge," writes Popper, "mistakenly took it for granted that there was only one kind of knowledge - knowledge possessed by some knowing subject". In this book, Popper argues that this "subjective knowledge" - knowledge held in an individual's mind - is not the only kind that exists; he proposes that we cannot fully understand the growth of knowledge unless we also take into account "objective knowledge".
Objective knowledge exists in the "logical contents of books, libraries, computer memories, and suchlike." It is information that can be accessed by an individual, yet is stored outside his memory.
Moreover, such knowledge is not just the sum total of many people's memory, even though it originates there: new problems can arise from the structure of objective knowledge that were there before any person actually became aware of them. As an example, Popper cites the discovery of prime numbers - "we discover prime numbers, and Euclid's problem of whether the sequence of prime numbers is infinite arises as a consequence." So there is a sense in which objective knowledge is autonomous - a world in which we can make voyages of discovery just as we can make geographical discoveries on our planet.
Popper argues that objective knowledge is much more important than has been realised; this has been obscured by the emphasis given by so many philosophers (such as Descartes, Locke, Berkeley, Hume, Kant and Russell) on the importance of our subjective beliefs, and the origin and basis of subjective knowledge. Although these theories originated with philosophers, such assumptions are now widespread and have been assimilated into popular thought. As an example, consider the high value that is placed upon intelligence, and the consequent attempts to quantify this. No. consideration seems to have been given in testing intelligence to objective knowledge, yet tests are clearly dependent upon the assumption of a common basis of objective knowledge (grammar, logic, arithmetic, semantics). It is assumed that the "background learning" required to solve test problems (understanding of language etc.) is static, and underlying this assumption is the idea that the "background learning" - subjective knowledge - is the only kind of knowledge.
To demonstrate the existence and significance of objective knowledge, Popper considers two thought experiments. Firstly, he asks us to imagine that "all our machines and tools are destroyed and all our subjective learning, including our subjective knowledge of machines and tools, and how to use them. But libraries and our capacity to learn from them survive. Clearly, after much suffering, our world may get going again." Secondly, he asks us to imagine the same situation, except that "this time, all our libraries are destroyed also, so that our capacity to learn from books becomes useless." It can be seen that the existence of information in books makes a crucial difference. This is a clever and beautifully simple argument on the distinction between subjective and objective knowledge, and the singular importance of the latter.
This is an interesting and well-argued book, severely critical of the subjectivist approach to knowledge. Other sections deal with the aim of science, indeterminism in physics and biology, and historical investigation. It is an exciting book, and will inspire the reader to thought and argument.