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http://dx.doi.org/10.1037/13642-005
The reader will recall the two kinds of threshold mentioned: the threshold where a sensation fades completely away, and the threshold where two impressions, although clear and strong, cease to be distinguishable. It is customary in psychology to draw a sharp line between these two kinds of phenomena, and to say that the disappearance of a sensation is quite different from the fading out of a difference between two sensations. In all probability, however, this is a mistake, and the two facts spring from the same source. The point at which a diminishing sensation seems to die away is probably not where its intensity becomes zero, but is merely the point at which it is no longer distinguishable from the nebulous background of sensations that are always with us. Because immediate sensible confirmation is dispensed with, we need not think that all the ordinary regard to the rules of evidence are abrogated and that we are free to believe what we please. There must still be sensible evidence for whatever is proposed in this field; but since the evidence must now be circumstantial, rather than direct, all the more need of putting it mercilessly to the test, of holding before ourselves alternative explanations, of weighing and sifting until we can convince ourselves that there is more reason to believe that some unseen factor is at work than that the causes of the phenomena are among the events we can observe. Neither credulity nor hardened unbelief will serve us here; we must demand the evidences, but respect them when they come. Such an attitude seems hardly to endanger the progress of psychology, but rather gives an added assurance that the study will progress. (PsycINFO Database Record (c) 2016 APA, all rights reserved)