The exorcist must proceed in his examination with the same objectivity and rigor as a doctor, so that no manifestation presented by the behavior of the patient is neglected.
He must avoid being drawn in by the first impressions, which may be misleading.
This critical examination has the objective of eliminating any possible natural explanation that he observes in the presumed diabolic manifestation.
Monsignor Maquart explains that there are a certain number of symptoms of possession that are also common in some diseases, such as the psychasthenia, hysteria, some forms of epilepsy, etc.
How can one tell the difference between a simple mental illness and possession by the demon?
It is here that other signs of possession – which do not have a natural explanation - become important: the ability to speak foreign languages unknown until then; the knowledge of facts happening in distant places; the revelation of unsuspected scientific talent; the show of exceptional physical strength, etc.
This diagnosis demands, at the same time, much objectivity and common sense, and also great faith. For, obviously, one cannot - under the excuse that the preternatural is an exception - negate in principle all and any demonic action. Nor can one proceed in such a way as to always find, at all costs, a natural explanation for the phenomena.