Tria sunt homini necessaria ad salutem: scilicit scientia credendorum, scientia desiderandorum, et scientia operandorum. - Thomas Aquinas

In ends there is found a twofold order, to wit, the order of intention and the order of execution, and in both orders there must be some first point. That which is first in the order of intention is a sort of principle moving the desire: take that principle away, and desire would have nothing to move it. The moving principle of the execution is that from whence the work begins: take away that moving principle, and none would begin to work at anything. Now the moving principle of the intention is the last end: the moving principle of the execution is the first step in the way of means to the end. Thus, then, on neither side is it possible to go on to infinity: because, if there were no last end, nothing would be desired, nor any action have a term, nor would the intention of the [3] agent rest. On the other hand, if there were no first step in the means to the end, no one would begin to work at anything, and deliberation would never terminate, but go on to infinity.

In deliberation we may hesitate; but a deliberated act must be performed swiftly.

[I]t is to be borne in mind, in regard to the philosophical sciences, that the inferior sciences neither prove their principles nor dispute with those who deny them, but leave this to a higher science; whereas the highest of them, viz. metaphysics, can dispute with one who denies its principles, if only the opponent will make some concession; but if he concede nothing, it can have no dispute with him, though it can answer his objections. Hence Sacred Scripture, since it has no science above itself, can dispute with one who denies its principles only if the opponent admits some at least of the truths obtained through divine revelation; thus we can argue with heretics from texts in Holy Writ, and against those who deny one article of faith, we can argue from another. If our opponent believes nothing of divine revelation, there is no longer any means of proving the articles of faith by reasoning, but only of answering his objections — if he has any — against faith. Since faith rests upon infallible truth, and since the contrary of a truth can never be demonstrated, it is clear that the arguments brought against faith cannot be demonstrations, but are difficulties that can be answered.