Thomas Aquinas

“ For it is natural to man to arrive at the intelligible truth through its sensible effects. Wherefore just as man led by his natural reason is able to arrive at some knowledge of God through His natural effects, so is he brought to a certain degree of supernatural knowledge of the objects of faith by certain supernatural effects which are called miracles. Therefore the working of miracles belongs to a gratuitous grace. ”

“ I answer that, Some miracles are not true but imaginary deeds, because they delude man by the appearance of that which is not; while others are true deeds, yet they have not the character of a true miracle, because they are done by the power of some natural cause. ”

“ True miracles cannot be wrought save by the power of God, because God works them for man's benefit, and this in two ways: in one way for the confirmation of truth declared, in another way in proof of a person's holiness, which God desires to propose as an example of virtue. In the first way miracles can be wrought by any one who preaches the true faith and calls upon Christ's name, as even the wicked do sometimes. In this way even the wicked can work miracles. ”