Believers say that the evidence is destroyed or that the absence of evidence isn't evidence of absence and that there is a convergence of evidence. There are rhetorical questions being used for the Holocaust.
No, they're not rhetorical questions.
If you had any familiarity with the historical disciplines whether in science, social science or the humanities, then you'll know that the overwhelming majority of information about the past is destroyed due to the ravages of time. Yet this has not stopped scientists and scholars reconstructing the past. Geologists can tell us about past cataclysms and upheavals from teh traces left in rocks. Palaentologists can tell us about extinct species from fossils. Linguistics can reconstruct dead languages, philology can reconstruct missing texts, archaeology can reconstruct entire civilisations from potsherds, and historians can reconstruct past events using a variety of types of evidence.
The main rule of thumb in history is that evidence closest to the events is
preferable. That does not mean that later evidence is dismissed, it simply means that depending on what one is asking then one may be forced to rely on other types of evidence. Most of all, it depends on what information has survived and been handed down to us.
Our knowledge of the Peloponnesian War is almost entirely derived from a single source, Thucydides. Archaeology cannot corroborate most of the events described in his account, and there are really few other written sources to do the same thing. There is unsurprisingly a great amount of disagreement over whether one can rely on Thucydides. But few would handwave away the entire account and dismiss the Peloponnesian War as a figment of his imagination. It does, after all, help us explain the rise of Athens in ancient Greece, which left other evidence and thus 'fits' with our expectations. We also have Herodotus writing in the period immediately beforehand. But we don't have a comparable account to Thucydides written by a Spartan.
That is an extreme example, but we also have more recent ones such as the destruction of virtually all Imperial German Army military records in 1945 by an RAF raid on Potsdam, which also wiped out much of the WWII Luftwaffe's records. In between 1918 and 1945, German military historians had written extensively on WWI using the now destroyed records. We do possess surviving collections of WWI German military records from state armies (Bavaria etc) and from captured documents - I have seen WWI documents captured by the British and found them very interesting. But the bulk of the material is
gone, forever, and we have to rely extensively on the renditions of this material written up in the 1920s and 1930s by manifestly biased nationalist historians who might well, as citizens of a defeated power, have good motive to lie.
The records generated by the Nazi regime are manifestly incomplete, and the files of many crucial agencies went up in smoke at the end of the war because they were deliberately destroyed by a regime on the brink of defeat. Only a handful of files of the Army's Quartermaster-General survive, the Luftwaffe records are fragmentary, and the main bulk of the records of the Party Chancellery were destroyed although some remain. In the 1980s, historians reconstructed the Party Chancellery's directives and circulars using the files of other agencies.
It is standard practice for military units to destroy files to prevent them falling into the hands of the enemy and thereby glean intelligence from them. Thus, there are very few files relating to the period of the retreat from Moscow since division after division reported in a later war diary period that they burned their intelligence department files or operational files during the retreat.
Much of the destruction was entirely deliberate and designed to cover up what the regime had done. And thus it is no surprise that the SS and Police records in particular are highly incomplete. There are only a handful of war diaries of individual police companies or police battalions extant, yet the Nazis fielded more than 100 police battalions. From other files you can often reconstruct what they did, but not always. The records of the concentration camps are similarly fragmentary. We have relatively complete sets for some camps, and virtually nothing for other camps.
Under these circumstances, insisting that something only happened if there is a contemporary Nazi document recording it would be a violation of every maxim and principle used in any historical discipline. It would be like insisting that dinosaurs existed only if an intact dinosaur could be found, instead of fossil bones.
However, there are enough records surviving that we can reconstruct and write the history of the Nazi regime to a level of detail which would make medievalists and many early modernists cry. Indeed, there is more written evidence available for the Holocaust than we are ever likely to see regarding many other mass exterminations in modern history, since the genocidal regimes in question destroyed many of
their records, too.
Moreover, the records that do survive permit us to assert with great certainty what happened. We do not need an extant Hitler order to say that Hitler ordered the extermination of the Jews if we have numerous documents from Himmler and other leading Nazis referring to a Hitler order, and many statements from Hitler right through to his Political Testament which corroborate this and disprove the claim that Hitler hadn't known. The sum total of pieces of contemporary written evidence relating to a Hitler order is in the many hundreds. They might leave us uncertain
precisely when an order was given, but they narrow the possibilities to within a very tolerable time-frame. Especially when contrasted with other evidence showing how policy was implemented.
Likewise, we do not need an extant Himmler order decreeing a stop to the gassings at Auschwitz to comprehend what happened at Auschwitz in the autumn of 1944. Transport and camp records indicate that there were no more selections upon arrival after the end of October 1944, and that there were no more mass selections inside the camp after the same date. Contemporary letters from SS doctors working at Auschwitz report their jubiliation that the whole nasty mess (I am paraphrasing) had been ended on the orders of Berlin.
Those sources, all of which are written contemporary documents, are sufficient to infer an order to stop gassing. One can infer who might have given the order by paying attention to the chain of command. Consilience alone would help identify the most probable candidate in the absence of any other evidence - most notably, the precedents of other similar orders on fairly important issues. Logically, a stop order could have been given by the camp commandant, by Gluecks at Amtsgruppe D, by Pohl at the WVHA, by Himmler or by Hitler. It is just about plausible that the RSHA hierarchy was involved, from Eichmann through to Mueller to Kaltenbrunner to Himmler. But even though the RSHA was heavily involved in the camps through the camp political departments and also organised transports to Auschwitz, it was not responsible for selections inside the camp, which were the province of the camp commandant's staff and the medical department. So it's more likely the camp-Amtsgruppe D-WVHA-Himmler hierarchy. And we can justifiably assume that most major decisions would have been taken higher up. So Himmler is very much the most probable candidate
even in the absence of any other evidence. Pohl might have actually issued the order, but it's unlikely that he would have done so without some discussion with Himmler. And Pohl had a track record of issuing orders signed by him which began 'on the orders of the Reichsfuehrer SS', which was a typical Nazi practice found with Hitler and other leading Nazi figures.
So when we encounter testimonial evidence given
immediately after the war that Himmler had indeed issued a stop order, this corroborates the already existing most probable interpretation of the "facts on the ground". In turn, other postwar testimonial evidence from inside Auschwitz corroborates the interpretation, since the staff and inmates at Auschwitz confirm that all of a sudden, selections stopped and the gas chambers began to be dismantled. By December 1944 we even have contemporary written evidence of a dismantling kommando assigned to tear down the crematoria. Was that decision taken locally? Could have been, but it doesn't seen very probable given all of the other evidence.
And it's the "all of the other evidence" bit which is important. Historians are not meant to dismiss entire categories of evidence just because cranks think that only certain types of evidence count. They are
routinely confronted by situations where no such evidence would ever have existed, or where it is known that evidence of a particular type which did exist, has been destroyed. Much of human history can only be known through evidence which is no different, indeed often far worse, than 1945-46 affidavits by leading Nazis.
If we had no postwar affidavit from Becher about a Himmler stop order, we would still be able to say, 'gassings at Auschwitz stopped, most probably on the orders of Himmler' (see: letter by Wirths, evidence of transports, testimonies of other SS men at Auschwitz etc). We might then have to rely on other later witnesses who confirmed the same point, to change the "most probably" to a reasonable certainty. (And note that the identity of who precisely ordered a stop to gassing is an entirely subsidiary issue - the evidence indicates gassings stopped, period.)
In such a situation - and I repeat, historical disciplines encounter them almost every day - then there will always be a residual element of uncertainty. But, and this is the other part that cranks never understand, you then have to reassess and reevaluate all of the relevant evidence, and produce a
better explanation of all the evidence. This is the principle known in philosophy as inference to the best explanation, sometimes as abduction, and the historical disciplines have to practice both so often they are one of the hallmarks of such disciplines. Historical dsiciplines, after all, cannot observe the past through an electron microscope, but only through the traces left by the past.
All this just sums up what is virtually instinctive to anyone who has training in a historical discipline.