An alien spaceship crashed in Roswell New Mexico in 1947

Assume that there never was a spaceship. What would then be the result of a search for evidence? What would that prove?

Loch Ness contains a reptile with a head the size of a bus.

Assume that there is no monster. What would then be the result of a search for evidence? You find nothing in the Loch but fish. Maybe it was hiding?

Loch Ness does not contain a reptile with a head the size of a bus.

Assume there is a monster. What would then be the result of a search for evidence? You might find it. That result would prove the statement wrong. -- It is a scientific statement.

This critter (just pulled from Loch Ness) is a fish.

Assume it is not a fish. It might, for example, have fur. That result would prove the statement wrong. -- It is a scientific statement.