It is highly likely that Daniel got the milk. It is highly likely that Greg is white. We doubt that Julius is a rhino. It is certain that if Greg is white, and Julius is a rhino, then Brian is a swan. Chances are about even that if Greg is white, and Julius is a rhino, then John picked up the apple. It is probable that if Daniel got the milk, and Greg is white, then Mary went to the office.
It is improbable that greg is white, or John picked up the apple.
This example is valid.