An engineer, a physicist, and a mathematician are on a train in Scotland.
The engineer looks out of the window, sees a black sheep standing in a field, and remarks, “How odd. All the sheep in Scotland are black!”
“No, no, no!” says the physicist. “Only some Scottish sheep are black.”
The mathematician rolls his eyes at his companions’ muddled thinking and says, “In Scotland, there is at least one field containing at least one sheep, at least one side of which appears to be black from this vantage point some of the time.”
A Buddhist monk overhears this exchange and remarks: “That is what you currently perceive. In each of us, there is a river of perceptions. Perceptions arise, stay for a period of time, and cease to be. Our perceptions are often erroneous and cause us to suffer and cause others to suffer. It is very helpful to look deeply into the nature of our perceptions, without being too sure of anything. When we are too sure, we suffer. When we ask ourselves, ‘Are you sure?“ we have a chance to look again and see if our perception is correct or not.”