Abba Benjamin used to say, “If our eyes were permitted to see the malignant sprites that beset us, we could not rest for a moment on account of them.” The air, the rabbis taught, is thicker with unseen things than the ground is with dust.
Abaye said, “They outnumber us. They surround us as the earthed-up soil surrounds our garden beds.” Rav Huna added, quoting the Psalms, “Every person has a thousand at his left side and ten thousand at his right” (Psalms 91:7). Rava completed the picture: “The crowding you feel in the houses of study, the weariness in your knees — that is them pushing in. They even tear our clothes by hustling against us.”
How would you prove they are there? Rava gave a practical test: sift ashes on the floor by your bedside before you sleep. In the morning you will see tracks, as though a flock of birds had crossed the room in the night.
And if you wanted to see the demons themselves? The rabbis gave a recipe — burn to ash the afterbirth of a firstborn black kitten whose mother was herself a firstborn black cat, then put the ash in your eyes. You would see them. But the rabbis add a warning: whoever looks on the unseen world with unprepared eyes loses something in himself. Better to know they are there, and keep walking.