Helge Kragh (geb. 1944): Aus dem Kapitel Anachronical and diachronical history of science, in: An Introduction to the Historiography of Science. Cambridge University Press, Cambridge, New York u.a. 1987, S.89-107:
Anachrone und diachrone Wissenschaftsgeschichte
According to the anachronical view, the science of the past ought to be studied in the light of the knowledge that we have today. ... Today, anachronical history of science is rarely a conscious historiographical strategy. On the contrary, there is a broad agreement about praising a non-anachronical ideal. Even so, in practice, anachronical history of science is widespread and difficult to avoid. ...
The diachronical ideal is to study the science of the past in the light of the situation and the views that actually existed in the past; in other words to disregard all later occurrences that could not have had any influence on the period in question. ... So, ideally, in the diachronical perspective one imagines oneself to be an observer in the past, not just of the past. This fictitious journey backwards in time has the result that the memory of the historian-observer is cleansed of all knowlwedge that comes from later periods. ... In this sense, one may say that there is a relativistic element in diachronical historiography. ...
The anachronically inclined historian, when dealing with William Harvey's famous discovery of the circulation of the blood (1628), will legitimate it by standing that, though it had certain speculative characteristics, Harvey's theory has proved to be an essentially correct description of the passage of the blood in the body. ... The diachronically inclined historian, dealing with the same subject and trying to put himself into the situation of a person working about the year 1640, will be more cautious in his evaluation of Harvey's discovery. In fact, Harvey was ridiculed at first and his theory of circulation was met with much opposition and scepticism during the first decades. The historian will be interested in how Harvey's work was received at the time, for example, in the criticisms directed at the theory by Gassendi and others. And he will draw attention to the support given to Harvey by mystics and alchemists (such as Robert Fludd and Elias Ashmole) on a definitely unscientific basis. While Fludd will appear in diachronical historiography as a key person in connection with Harvey, he might not be mentioned at all in anachronical historiography. In anachronical historiography the subject matter of history of science is the same as the subject matter of science. ... Accordingly, science becomes a phenomenon that is bound to make progress in the direction of truth. ...
Diachronical historiography can only be an ideal. The historian cannot liberate himself from his own age and cannot completely avoid the use of contemporary standards. During the preliminary study of a specific period one cannot use the period's own standards for evaluation and selection; for these standards form part of a period that has not yet been studied and they will only gradually be revealed. In order to have any kind of view at all of one's subject one has to wear glasses; and these glasses must, unavoidably, be the glasses of the present. ...
In many cases it will be the obvious thing to do to use modern knowledge in the analysis of a historical event. By so doing one may be led to interesting questions that could not be formulated on a purely diachronical basis. Thus, in the opinion of most historians, it is interesting to ask why the Greeks did not discover the irrational numbers, a problem central to the understanding of the foundation crisis of Greek mathematics. But a question like this can obviously only be put by somebody who knows that rational numbers can be extended with irrational numbers in the way that happened much later. ...
We conclude that in practice the historian is not confronted with a choice between a diachronical or an anachronical perspective. Usually both elements should be present, their relative weights depending on the particular subject being investigated and the purpose of the investigation. The historian of science has to be a person with the head of a Janus who, at the same time, is able to respect the conflicting diachronical and anachronical points of view.