Like a succession of dazzling but useless phantasmagoria

From History of Medical Delusions by Worthington Hooker and published in 1850.

In his introductory paragraph, Hooker describes what I refer to as the frontiers of knowledge. As we conquer new terrain, we often forget what is behind us and we take for granted that before us, not stopping to acknowledge the realm of that which we do not know but think we do.

The lessons which are taught by the history of past delusions are slowly learned by the medical profession, and still more slowly by the community at large. In the progress of human knowledge medicine has not been disencumbered of error so rapidly as the other sciences have been. So little does it bear the character of an exact science, especially in its Therapeutics, and so prone are men to conjecture and theorize where they cannot know, that the errors of the past on this subject have very generally failed to guard effectually against errors in the future. The history of medicine, therefore, presents to our view a succession of errors, standing out in bold prominence; each one having, as it rose to its ascendancy, supplanted some favorite error which preceded it. Truth, however, let it be remembered, has been all the time more and more developed, by a constant accession to the facts and established principles of our science. And these facts and principles remain as permanent acquisitions, the property of the profession through all time; while its array of baseless, but splendid theories and doctrines has passed away, like a succession of dazzling but useless phantasmagoria.