When Absence of Evidence is Evidence of Absence
It is popularly believed that absence of evidence is not evidence of absence - or, to present what appears to be the Talmudic equivalent, לא ראינו אינו ראיה. Often, however, this is not true, and absence of evidence is indeed evidence of absence.
Let's take Bigfoot - the large, hairy, ape-like creature that is said to stalk the forests of north-western America. There is no actual evidence for its existence. Now, you might think that this does not mean that there is any evidence that Bigfoot does not exist. But it does indeed mean that.
The reason is that if Bigfoot existed, there couldn't just be one or even a few of them. No species exists as just a few individuals. There would have to been, over history, hundreds of thousands of them. And while it is certainly possible that a handful of individuals could exist without leaving any evidence for others to find, it is inconceivable that hundreds of thousands could exist without leaving any evidence behind. The absence of any evidence for Bigfoot is indeed evidence that Bigfoot has never existed.
So absence of evidence is indeed evidence of absence if the phenomenon would be expected to produce evidence. It is important to grasp this point, because it has significant ramifications for discussions about Torah - in particular, the history of rabbinic scholarship.
Recently I have been involved in various online Torah discussions in which people asked for rabbinic sources to back up a certain idea. The presumption appeared to be that if you can find some source - any source - then it shows that the idea is a legitimate, normative idea in Judaism. And if you can't - well, it still could well be that the idea is normative, and we just haven't found the source yet.
But you have to consider the situation. If the idea is indeed normative, then surely you would expect it to be widely discussed by Chazal and certainly the Rishonim. If there is no discussion of it, then this is evidence that they did not believe the idea to exist. If there are only one or two sources discussing it, then this is evidence that it is not normative.
(Yes, I am aware of the irony that during the Great Science-Torah Controversy, this is exactly what the Gedolim accused me of doing, with regard to sources claiming that Chazal were fallible in scientific matters. But the point is that they got it exactly backwards. The notion that Chazal were fallible in science is not an aberrant view, but is in fact that of a major school of thought in rabbinic history. Whereas the idea that Chazal knew everything about the world from ruach hakodesh is not found in any early sources, and is countered by countless Gemaras and other sources.)
This is a point of general importance, but it is also of particular importance with regard to the topic that I saw being discussed. This was the notion that you can learn Torah and designate the reward/benefits to the spiritual bank account of anyone that you like, as long as you mention their name. As I discussed in my monograph "What Can One Do For Someone Who Has Passed Away?" there is no early source for such a concept; in fact, the earliest authorities to discuss such things explicitly state that there is no such concept. You can't do mitzvos and designate the reward to be sent to whomever you want. So, when people are failing to find sources for such a notion, they should not conclude that the sources are out there and they just haven't found them yet. Rather, the correct conclusion is that this is not a normative, classical idea in Judaism.