Is there a "just noticeable difference"?

Working within a branch of experimental psychology called psychophysics, scholars argue about "just noticeable differences," thresholds of perception, and related postulations.

Consider the volume of sound as an issue. It seems intuitively there should be sounds that are different from one another in an objective sense (mechanically measurable) in that one is louder than the other, but that are not perceived as different by human beings.  After all, we did not evolve as mechanisms for the precise measurement of sound,  We evolved, to be simple about it, to survive and reproduce. That does not require ideally good distinctions in these matters. 

So there should be some just noticeable difference between sound A and sound B as to volume, such that if I hear any pair closer in volume to each other than those two, I will perceive them as identical. Right?

Likewise with weights? Put one object in my left and put another in my right and ask me to tell you which is heavier.  At some point in your series of experiments of this sort you will have reached JND as to weight. Beyond that, I, the subject, will perceive them as the same weight even if they are not. Fair enough? 

Well, as plausible as it seems, that line of thought is misleading.  Experiments intended to find the JND ended up discovering the subliminal nature of much perception. Suppose you coax me, even after I have said "both weights feel the same to me." You coax me to guess.  I will consistently guess right more often than wrong.  There is a conscious JND, but subliminal perception of the differences continues. 

I'm not discussing anything new. These are experimental results from more than a century ago. A classic paper on this subject was published in the Proceedings of the National Academy of Sciences in 1916, authored by Charles Peirce (of pragmatism/pragmatics fame) and Joseph Jastrow (who was well known at the time as a debunker of parapsychological claims in that age of Harry Houdini). 

JND and the search for that threshold retains its fascination, IMHO. 

James' discussion of psychophysics in PRINCIPLES OF PSYCHOLOGY (1890) represents an earlier state of play. Had he lived long enough to read up on the Peirce-Jastrow findings we can say, on the basis of that discussion, that he would not have been terribly surprised.  Indeed, in some of his later writings he makes use of the then-emerging concept of subliminal perception. 

James' use of the concept is distinct from Jastrow's use. Jastrow, as a debunker of parapsychology, was able to rely on subliminal knowledge as an alternative explanation of some of the facts invoked by advocates of paranormal phenomena. You don't KNOW how you know that the object in your left had is a little heavier than the object in your right. But it is and you regularly guess accurately.  Aha, a paranormalist might say, maybe a ghost is telling you! Or, just maybe, Jastrow would respond, one part of your nervous system is telling another, slipping it notes under the table, so to speak.  

James, though, used the idea of subliminal knowledge to support the possibility of a more direct mind-to-mind connection than physical modes of communication suggest. The reality of it might be a tide, going out, that shows us surprises beneath the water. What we know but didn't know about volumes and weights shows up almost as soon as the waters begin to recede. What we can access of one another's mind, and the possibility we all have one mind, becomes visible only with a further recession but is in the same line of development.   

Ah, the depths to which one is led by a simple question such as whether there is a just noticeable difference.