The possibility of the transference of thought, apart from the recognized channels of sensation, has been too frequently discussed, with the suppressed or unconscious assumption that our knowledge of the means whereby we ordinarily and normally, consciously and unconsciously, convey to others some notion of what is passing in our own minds, is comprehensive and exhaustive. Nothing could be farther from the truth. Whenever a mode of perception, no matter how limited or apparently trivial, has been thoroughly investigated, there have been discovered, or at least suggested, unrecognized possibilities of its use and development. And no result of experimental inquiry is more constantly illustrated than the extent to which inferences from sensations and the exercise of faculties may proceed without arousing consciousness of their existence. Many color-blind persons remain quite ignorant of their defect; and it was only after the description of his own notable deficiencies by Dalton (in 1794) that the general prevalence of color-blindness became recognized. The fact that a portion of every one's retina is as blind as his finger-tip escaped observation until about two centuries ago; and this because the normal use of our eyes does not present the conditions of its easy detection; and for a like reason we persistently refuse to see the double images that are constantly formed upon our retinæ. With the same unconsciousness that we receive sensations and draw inferences from them, do we give to others indications of what is going on in our minds, and read between their words and under their expressions what "half reveals and half conceals the thoughts that lie within." It is important to emphasize the serious limitations as yet attaching to our knowledge of the detailed possibilities of normal perception and inference, in order to realize the corresponding hesitancy with which we should regard any series of facts, no matter how apparently inexplicable, as evidence of a supernormal kind of mental telegraphy.