A block officer writes: 'The training we give the village health volunteers works. We call a volunteer trained only when she is able to do the work; and every volunteer we have called trained is able to do the work.' The officer's reasoning is most open to criticism because

The officer's second sentence cannot fail. He has defined 'trained' as able to do the work, so reporting that everyone he calls trained can do the work is reporting the definition back, and a sentence that no possible state of the world could falsify carries no information about the training. The figure that would settle the question is a different one — of every hundred volunteers who sit through the course, how many come out able to do the work — and the officer's way of counting keeps it permanently out of sight. How many have been called trained, how the volunteers were assessed and how long the course runs would all be worth knowing, but none of them is the reason the argument fails.