Weber  "Objectivity of Social Science and Social Policy" 

Nothing, however, is more dangerous than the confusion of theory and history stemming from naturalistic prejudices. This confusion expresses itself firstly in the belief that the "true" content and the essence of historical reality is portrayed in such theoretical constructs or secondly, in the use of these constructs as a procrustean bed into which history is to be forced or thirdly, in the hypostatization of such "ideas" as real "forces" and as a "true" reality which operates behind the passage of events and which works itself out in history. 

This latter danger is especially great since we are also, indeed primarily, accustomed to understand by the "ideas" of an epoch the thoughts or ideals which dominated the mass or at least an historically decisive number of the persons living in that epoch itself, and who were therefore significant as components of its culture. Now there are two aspects to this: in the first place, there are certain relationships between the "idea" in the sense of a tendency of practical or theoretical thought and the "idea" in the sense of the ideal-typical portrayal of an epoch constructed as a heuristic device. An ideal type of certain situations, which can be abstracted from certain characteristic social phenomena of an epoch, might--and this is indeed quite often the case--have also been present in the minds of the persons living in that epoch as an ideal to be striven for in practical life or as a maxim for the regulation of certain social relationships. This is true of the "idea" of "provision" and many other Canonist doctrines, especially those of Thomas Aquinas, in relationship to the modem ideal type of medieval "city economy" which we discussed above. The same is also true of the much talked of "basic concept" of economics: economic "value." From Scholasticism to Marxism, the idea of an objectively "valid" value, i.e., of an ethical imperative was amalgamated with an abstraction drawn from the empirical process of price formation. The notion that the "value" of commodities should be regulated by certain principles of natural law, has had and still has immeasurable significance for the development of culture--and not merely the culture of the Middle Ages. It has also influenced actual price formation very markedly. But what was meant and what can be meant by that theoretical concept can be made unambiguously clear only through precise, ideal-typical constructs. Those who are so contemptuous of the "Robinsonades" of classical theory should restrain themselves if they are unable to replace them with better concepts, which in this context means dearer concepts.

Turner, "The Continued Relevance of Weber's Philosophy of Social Science"

...For Weber, ’logic’, which might be taken by him to include calculation and decision-theory, was non-valuative, in contrast to the domain of described facts of the historical sciences, in which a ’valuative’ element entered. What is valuative is, for Weber, what is ours: logic is everyone’s.