Abba Benjamin says, "If our eye were permitted to see the malignant
sprites that beset us, we could not rest on account of them." Abaii has
said, "They out-number us, they surround us as the earthed-up soil on
our garden-beds." Rav Hunna says, "Every one has a thousand at his left
side and ten thousand at his right" (Ps. xci. 7). Rava adds, "The
crowding at the schools is caused by their pushing in; they cause the
weariness which the Rabbis experience in their knees, and even tear
their clothes by hustling against them. If one would discover traces of
their presence, let him sift some ashes upon the floor at his bedside,
and next morning he will see, as it were, the footmarks of fowls on the
surface. But if one would see the demons themselves, he must burn to
ashes the after-birth of a first-born black kitten, the offspring of a
first-born black cat, and then put a little of the ashes into his eyes,
and he will not fail to see them," etc., etc.
THE TALMUD, _Berachoth_, fol. 6, col. 1.