p There is a quite rich literature on the principle of observability in physics. Physicists and philosophers have published much analysing, on the philosophical plane, matters pertaining to this principle.^^1^^ Their work draws attention to aspects as yet little studied of the principle of observability. We should perhaps recall here that several authors associate this principle with Mach’s philosophy. Is that right? We shall discuss the point below. Here we shall just note that the ’principle of observability" has to some extent become similar to the affirmation of the ’disappearance of matter’ that diverted many scientists, at the time of discovery of radium and electrons, from the materialist path, the real meaning of which was revealed by Lenin in Materialism and Empiric-criticism.^^2^^

p In this chapter we shall consider the theory of knowledge and methodological problems that are associated with the principle of observability. Let us begin with some definitions.

p Observation, of course, is a cognitive activity of man associated with purposeful perception of objectively real objects and phenomena; in other words, observation of the objects and phenomena of nature would be impossible without their direct or mediated (through devices or means of observation) effect on the human sense organs. From this materialist point of view the concept ‘observability’ means the possibility of observing objects and phenomena of 96 nature that exist objectively and independent of the human mind. Objects exist objectively, of course (and are therefore cognised), in various relations to other objects and in their development, but we shall leave issues of the cognition of these relations and of the role of observation in cognition.

p The concept of observability is also employed in modern physical literature with a more special meaning, namely, in the sense that physical statements about objects, phenomena, properties, and magnitudes that are recognised as observable must satisfy certain requirements (which will be discussed below). Our job is to analyse and substantiate these requirements, since the principle of observability means just them. Let us consider the statements of authors in which the principle of observability is formulated in one way or another.

p According to Einstein, the concept ‘simultaneous’ ’does not exist for the physicist until he has the possibility of discovering whether or not it is fulfilled in an actual case’.^^3^^

p Paul Langevin wrote that ’theory should not, as far as possible, introduce anything that has no experimental significance and that does not correspond at least to an imaginable if not easy experiment’.^^4^^

p In The Feynman Lectures on Physics one can find the following statements: ’Another thing that people have emphasized since quantum mechanics was developed is the idea that we should not speak about those things which we cannot measure. (Actually, relativity theory also said this.) Unless a thing can be defined by measurement, it has no place in a theory—Just because we cannot measure position and momentum precisely does not a priori mean that we cannot talk about it. It only means that we need not talk about them.’^^5^^

p F. A. Kaempffer has written: ’Quantum mechanics purports to be a description of physical reality which deliberately eliminates from theory all features not demanded by experiment.’^^6^^

p Some authors confuse the epistemological question of objective reality and its cognition by man with the content of the principle of observability. This strikes one sharply in Eddington’s works. He stated, for instance, that when the physicist formulated the laws of mechanics, he was dealing not with ’wholly objective particles and wholly objective 97 behaviour (of these particles) but with their observed behaviour, with ’properties imposed by our procedure of observations’.^^7^^ For Eddington physical knowledge was essentially ’observational knowledge’ allegedly obtained by ’scrutinising the frame of thought’, ’by the observer’s sensory and intellectual equipment’^^8^^ used for observation (and not through deeper and deeper generalisation of facts obtained by means of observation). It was by this a priori way that we allegedly obtained knowledge of the fundamental laws and constants of physics (e.g. the speed of light in vacuum) which, in Eddington’s view, were ’wholly subjective’ and ’can be discovered a priori’.^^9^^

p The principle of observability actually has nothing in common with the problem of the relation between the subjective and the objective as resolved in the idealist manner. In a (closed) physical theory only those statements are admissible that are substantiated in one way or another or can be substantiated by experiment (observability in principle); those statements that cannot be substantiated by experiment are excluded from the theory. That is the meaning of the principle of observability.

p It is customary to cite the following confirmations of the usefulness of this principle. The critical analysis Einstein made of simultaneous events observable in principle in various places enabled him to arrive at relativistic conceptions of space and time. Similarly, Heisenberg overcame the difficulties in Bohr’s atomic model when he excluded the position of the electron in the orbit and its angular momentum as unobservable; he created matrix mechanics, the preliminary form of quantum mechanics, relying solely on the frequencies and intensities of spectral lines observable in experiment.

p Is it true that both leading theories of modern physics owe their origin just to the observability principle? The answer to that must be above all sought within the frame of physics itself and not on the basis of speculative conjectures as Eddington in essence suggested.  [97•*  One must not confuse the observability principle with the epistemological 98 statement about the relation between the subjective and the objectively real.

p Dirac wrote that ’science is concerned only with observable things’.^^10^^ Schrodinger expressed the same idea, but with a reservation, writing: ’We can never say what actually is ... but simply what will be observed in a particular concrete case. Must we be always satisfied with that...? In principle, certainly. In principle the demand that exact science should in the last analysis strive only to describe the actually observed is not at all new. The question is simply whether we must, from now on, renounce linking the description we used to use to a lucid hypothesis about how the world is really constituted. Many would already proclaim this rejection today, but I believe that we would thereby make things a little too easy.’^^11^^

p Dirac himself, strictly speaking, did not attach any epistemological meaning to his statement. This can be seen from his continuation of the citation above: ’We can observe an object only by letting it interact with some outside influence.’^^12^^

p Schrodinger, on the other hand, already ascribed an epistemological meaning to his statement, although he did not develop the appropriate argument.

p Heisenberg expressed himself clearly and unambiguously on the epistemological plane. According to his point of view, when we speak of modern exact science’s picture of nature, it is no longer a matter of ’a picture of nature, but of a picture of our relations to nature’.^^13^^ He assumed that when man observed nature, he was dealing not with nature itself but with nature considered through the prism of problems posed by man.

p We discussed Heisenberg’s point of view in detail in Chapter II, and are not going to dwell on it again here. We would simply like to stress that in their statements of a philosophical character both Eddington and Heisenberg in essence separated observation as a necessary aspect of the cognitive process from cognition as a whole, making an absolute of abstract thought, isolating it from objectively real nature and matter. When man observes nature and creates scientific theories he is, in fact, dealing with nature; human knowledge is relative, but this relativity is not absolute and is overcome to a greater and greater extent as cognition (knowledge), reflecting eternally developing nature, progresses.

p In a complete physical theory verified by experience its statements and .concepts reflect the material world; these concepts and statements, having been verified by experience, are true and correspond to objective reality. In the development of physical science, however, situations arise such that in certain conditions (e.g. when the researcher comes across a new sphere of phenomena in his experiments to which a given physical theory is not in essence applicable, but that has not yet been brought out by physics) some of the concepts and statements of the given complete theory do not correspond to objective reality. A new experiment disproves the assertion that a certain concept or statement of this theory corresponds to objective reality; which means that this concept or statement ceases to be true.

p The question is asked whether physics is right to admit statements and concepts on which experiment seems to give a dual answer as to whether they correspond to objective reality; and if it is right what is the epistemological content of this ‘admission’? Physics faces such questions when a new theory grows out of an old one, and the latter becomes a special, limiting case of this new, more general theory.

p The observability principle, as we shall see later, has something to do with the question posed; it also correspondingly concentrates the necessary material for answering it. If the question is formulated more rigorously, then it is a matter of paradoxical situations and of how to resolve the emerging paradoxes. The principle is also one of the methods of resolving paradoxes of this kind. The results of applying it do not so much reinforce statements (and theories) already known in physics as lead to new statements (and theories) not yet known to science. There were no such theoretical methods in the science of the classical period, and could not be, at least in their explicit form, because observed phenomena were explained in the final analysis at that time by a mechanical macroscopic model, and such an explanation was regarded as the only one possible that did not give rise in principle to any logical ambiguities, although engendering practical difficulties in complicated cases.

Even now individual authors voice serious doubts with respect to the objective substantiation and heuristic value of the principle of observability.^^14^^ It is said, in particular, that the principle does not answer why, in some cases, the 100 Emacs-File-stamp: "/home/ysverdlov/leninist.biz/en/1979/DMP383/20090805/199.tx" unobservables (like the trajectory of an electron in an atom) have to be excluded and in other cases are permitted (for example, wave functions); in some cases they are harmful (e.g. absolute simultaneity), and in other cases may play a necessary and positive role (e.g. this same wave function)?¹⁵ It will become clear from what follows, however, that the situation is not quite that hopeless. As we see it, the epistemological and methodological aspects of the principle of observability should not be considered independently of the questions associated with the application of dialectics to cognition and its development.