Knowledge is difficult to pin down. It has been described as a justifiable belief in something that is, in fact, true.

But even that can fail.

Imaging the driver of a vehicle that collects milk from dairy farms. Driving to a farm he sees a dairy cow in a field where she ought not to be. He tells the farmer that one of his cows has escaped and is in the field with the small wood in it.

The farmer is very busy but he drives to the field and sees the cow in the distance. She seems to be OK so he carries on and decides to collect her later. When he gets back to the farm he sees that the cow has come home on her own.

Now the farmer believes that the cow was in the field. He is justified in this belief because he saw what he thought was the cow when he drove by. The cow really WAS in the field but was, in fact, hidden in the wood. Some strange individual had placed a life sized cardboard cutout of the cow in the field and it was that which the farmer saw.

Can you say that the farmer KNEW the cow was in the field? If it had been somewhere else he would still have believed it was in the field.