'No cause,' when you consider the case in the abstract, but how if it be taken in the concrete, a man going to look for water in a region infested with robbers?
In speaking of a 'necessary cause' St Thomas is in fact thinking of a physical cause which is not likely to be counteracted, or to have any of its requisite conditions fail, e.g., the rotation of the earth producing sun-rise. In speaking of a contingent cause, -- so far as the phrase may be used without bringing free will into the field, -- he has in view a physical cause, the action of which may readily be counteracted by the interference of other physical causes, or may fail of effect because some one of its many requisite conditions is not present. A contingent physical cause, uninterfered with and having all its conditions present, works as a necessary cause.
Of God and His Creatures: 3.94