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. 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.
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. 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. We conclude that in practice the historian is not confronted with a choice between a diachronical or an anachronical perspective.