An aftereffect illusion of the tactile senses. Water at exactly the same temperature will feel completely different to your touch.
MATERIALS NEEDED: One bowl filled with very hot water. One bowl filled with room temperature water. One bowl filled with extremely cold water
WHAT TO DO: Arrange the three bowls of water with the different temperatures in a row of three so that the bowl with the room temperature water is in the middle. Place one hand in the cold water bowl and the other hand in the hot water bowl. It should feel extremely uncomfortable! After about three minutes take both hands out of the bowls and place them simultaneously in the middle bowl.
Does the temperature of the water feel the same to each hand. Or does it feel different? How does the temperature of the water feel to the hand that was formerly in the hot water? How does the temperature of the water feel to the hand that was formerly in the cold water? Why would it feel different? After all, the water is the same temperature!
EXPLANATION: The temperature illusion, like all aftereffect illusions, is due to the fatiguing of nerve cells firing a continual and constant rate. When the stimulus is removed the nerve cells continue to fire for a short while, even though there is no stimulus present.
This illusion was the basis for an extended discussion on the subject of reality by the famous French mathematician and philosopher Rene Descartes (1596-1650). He concluded from this experiment and other observations that trusting your sense was not the way to know reality. True reality comes only from ideas, including the idea of doubting one’s senses. Was Descartes right?