When scientific knowledge becomes scientific discovery: the disappearance of classical conditioning before Pavlov

In the nineteenth century, scientific materials in experimental physiology changed dramatically. In this context, phenomena that had been widely accepted were lost, sometimes to be reintroduced later as "discoveries." I describe the loss of the phenomenon of classical conditioning, later rediscovered by Ivan Pavlov. In 1896, Austrian physiologist Alois Kreidl demonstrated experimentally that animals anticipate the occurrence of food that is cued by a variety of stimuli. Kreidl stated, moreover, that the fact that animals can be called to food had been widely known to science since the 1830s. I describe Kreidl's work and discuss several factors that may have led to the disappearance of conditioning prior to its rediscovery by Pavlov.