Many of you are forgetting that collective knowledge is merely the notion of a result leading there. While we have made advances or such in the sciences, what we have before us is merely a particular paradigmatic stage in that field.
In other words, we have the knowledge, documentation, science, of our particular era. Though it might be claimed that there exists a corpus of knowledge which has existed beforehand that has led to our stage, as if to say we have a clear hindsight of our predecessors - and on top of that, our own modern innovations.
The idea here is that history is viewed only from the perspective of the present, a historical analysis today of bloodletting as a summary and transition or merely a stage in evolution to our current state of medicine is just that, a myth and story to give our practice a grounding in a kind of history and tradition. For the common man, that is for those who are not historians of medicine, it is nothing more than the knowledge the Greeks had of their own creation story (Prometheus and friends). The fact that "Some historian" knows exactly what has transpired does not add to the collective intelligence, or even the collective knowledge of civilization. The only thing we actually have over other, older generations is the general knowledge that we have more general knowledge - the perception of history and evolution from a collective perspective has been the same as it has always been in form, it is only the substance which has changed.
The common knowledge that there was once a medical treatment involving bloodletting and mercury, but that it is now known to be damaging is not much different from the common knowledge of the Bible that once people worshiped idols, but we know better. Surely one can interject that there has been documentation, scientific documentation relating to bloodletting etc. whereas an equivalent does not exist for idolatry; yet what this entails is merely the notion that "It is known", the form itself is appealed to - we never counter by stating exactly what it is which is known.