If any difficulty is found in respect of an existing law or an accepted truth, the law is challenged immediately.
First of all a hypothesis is framed to find out the cause of the difficulty. The hypothesis that is framed to solve the problem involved in case of a law, is known as adhoc-hypothesis.
An adhoc hypothesis remains adhoc till it is either confirmed or rejected. For example, the path of Uranus was decided as per the law of gravitation. Later on some deviations were marked.
In course of finding out the cause of deviation it was supposed that the deviation might be due to the presence of some other planet. In fact Bourvord discovered the planet Neptune through this method.