The claim is made that the adept at crystal-gazing can determine by the apparent difference in proximity of the visions whether they refer to the present or to a more or less remote past or future, that is to say, are nearer or farther removed in time from the period when the vision appears. The distinction between past and future is admitted to offer greater difficulty and a decision as to this point must depend upon a kind of intuitive and undefined impression on the part of the scryer. Those who have made a sympathetic study of crystal-gazing recognize that the “visions” seen in or on the crystal differ according to the mental and psychic temperament of the scryer. Two broad distinctions are sometimes established, the one class comprising those whose mental attitude is a “positive” one while the second class includes the “passive” subjects. In the former case the crystal visions are more apt to be symbols denoting some past or future event than a clear picture of the event itself, the mentality of the “positive” subject being, perhaps, too strong merely to mirror the image cast upon it. Instead of so doing it transforms the impression received from this image into some symbolic form. This process is not, however, consciously done, but the scryer of this type is supposed nevertheless to have an instinctive appreciation of the fact that what he sees is purely and simply a symbol, and he proceeds to interpret this in accord with certain generally received rules, or in accord with his own personal experience. The passive subject on the other hand is more apt to see a clear and definite picture of the persons or events revealed to him. Sometimes that picture is distinctly perceptible on or about the surface of the crystal, while at other times the visual perception will be rather indefinite and clouded, although accompanied by a strong mental impression in itself equivalent to that which would have been induced by an actual and objective vision.[316] The proper use of the crystal is the prime factor in the art of scrying and great attention is paid to this point by all those who treat seriously of the subject. Among other things they recognize that freedom from pain, or even from a sense of physical discomfort, is quite essential, for the mind must assume a purely passive and receptive attitude, and not be forced to take cognizance of bodily discomfort. Moreover the nervous system must be in repose, for which reason a reasonable time should be allowed to lapse after taking a meal, before trying for crystal visions.[317]