"The Logic of Scientific Discovery" by Karl Popper,1959

Logik der Forschung is a 1934 book by Karl Popper.
It was originally written in German, but reformulated in English by Popper himself some years later, to be published as The Logic of Scientific Discovery in 1959. This forms the rare case of a major work to appear in two languages, both written by the author instead of being translated.
Sir Peter Medawar called it "one of the most important documents of the twentieth century."
In it, Popper argued that science should adopt a methodology based on falsification, because no number of experiments can ever prove a theory, but a single experiment can contradict one. Popper holds that empirical theories are characterized by falsifiability.
In 1946 he moved to England to teach at the London School of Economics, and became professor of logic and scientific method at the University of London in 1949. From this point on Popper's reputation and stature as a philosopher of science and social thinker grew enormously, and he continued to write prolifically—a number of his works, particularly The Logic of Scientific Discovery (1959), are now universally recognised as classics in the field. He was knighted in 1965, and retired from the University of London in 1969, though he remained active as a writer, broadcaster and lecturer until his death in 1994.

Scientific Knowledge, History, and Prediction

At a very general level, Popper argues that historicism and holism have their origins in what he terms ‘one of the oldest dreams of mankind—the dream of prophecy, the idea that we can know what the future has in store for us, and that we can profit from such knowledge by adjusting our policy to it.’ (Conjectures and Refutations, 338). This dream was given further impetus, he speculates, by the emergence of a genuine predictive capability regarding such events as solar and lunar eclipses at an early stage in human civilisation, which has of course become increasingly refined with the development of the natural sciences and their concomitant technologies. The kind of reasoning which has made, and continues to make, historicism plausible may, on this account, be reconstructed as follows: if the application of the laws of the natural sciences can lead to the successful prediction of such future events as eclipses, then surely it is reasonable to infer that knowledge of the laws of history as yielded by a social science or sciences (assuming that such laws exist) would lead to the successful prediction of such future social phenomena as revolutions? Why should it be possible to predict an eclipse, but not a revolution? Why can we not conceive of a social science which could and would function as the theoretical natural sciences function, and yield precise unconditional predictions in the appropriate sphere of application? These are amongst the questions which Popper seeks to answer, and in doing so, to show that they are based upon a series of misconceptions about the nature of science, and about the relationship between scientific laws and scientific prediction.

His first argument may be summarised as follows: in relation to the critically important concept of prediction, Popper makes a distinction between what he terms ‘conditional scientific predictions’, which have the form ‘If X takes place, then Y will take place’, and ‘unconditional scientific prophecies’, which have the form ‘Y will take place’. Contrary to popular belief, it is the former rather than the latter which are typical of the natural sciences, which means that typically prediction in natural science is conditional and limited in scope—it takes the form of hypothetical assertions stating that certain specified changes will come about if particular specified events antecedently take place. This is not to deny that ‘unconditional scientific prophecies’, such as the prediction of eclipses, for example, do take place in science, and that the theoretical natural sciences make them possible. However, Popper argues that (a) these unconditional prophecies are not characteristic of the natural sciences, and (b) that the mechanism whereby they occur, in the very limited way in which they do, is not understood by the historicist.

What is the mechanism which makes unconditional scientific prophecies possible? The answer is that such prophecies can sometimes be derived from a combination of conditional predictions (themselves derived from scientific laws) and existential statements specifying that the conditions in relation to the system being investigated are fulfilled. Schematically, this can be represented as follows:

[C.P. + E.S.]=U.P.
where C.P. = Conditional Prediction; E.S. = Existential Statement; U.P. = Unconditional Prophecy. The most common examples of unconditional scientific prophecies in science relate to the prediction of such phenomena as lunar and solar eclipses and comets.

Given, then, that this is the mechanism which generates unconditional scientific prophecies, Popper makes two related claims about historicism: (a) That the historicist does not in fact derive his unconditional scientific prophecies in this manner from conditional predictions, and (b) the historicist cannot do so because long-term unconditional scientific prophecies can be derived from conditional predictions only if they apply to systems which are well-isolated, stationary, and recurrent (like our solar system). Such systems are quite rare in nature, and human society is most emphatically not one of them.

This, then, Popper argues, is the reason why it is a fundamental mistake for the historicist to take the unconditional scientific prophecies of eclipses as being typical and characteristic of the predictions of natural science—in fact such predictions are possible only because our solar system is a stationary and repetitive system which is isolated from other such systems by immense expanses of empty space. The solar system aside, there are very few such systems around for scientific investigation—most of the others are confined to the field of biology, where unconditional prophecies about the life-cycles of organisms are made possible by the existence of precisely the same factors. Thus one of the fallacies committed by the historicist is to take the (relatively rare) instances of unconditional prophecies in the natural science as constituting the essence of what scientific prediction is, to fail to see that such prophecies apply only to systems which are isolated, stationary, and repetitive, and to seek to apply the method of scientific prophecy to human society and human history. The latter, of course, is not an isolated system (in fact it's not a system at all), it is constantly changing, and it continually undergoes rapid, non-repetitive development. In the most fundamental sense possible, every event in human history is discrete, novel, quite unique, and ontologically distinct from every other historical event. For this reason, it is impossible in principle that unconditional scientific prophecies could be made in relation to human history—the idea that the successful unconditional prediction of eclipses provides us with reasonable grounds for the hope of successful unconditional prediction regarding the evolution of human history turns out to be based upon a gross misconception, and is quite false. As Popper himself concludes, "The fact that we predict eclipses does not, therefore, provide a valid reason for expecting that we can predict revolutions."