perception

The fact that we perceive something to be true does not make it true.

For example, we perceive that the stars revolve around the earth.

Or that the ocean ends on the horizon.

Or that magician saws a woman in half.

So perception can be an illusion.

On the other hand, as Mark Twain noted, if it looks like a duck, walks like a duck and quacks like a duck – it's a duck.