Reason, one can form various notions of the word reason .

1. One can mean simply and in the broadest sense that natural faculty with which God endowed men to know truth, whatever light it follows, and to whatever class of subjects it applies.

2. One can mean by reason this same faculty considered, not absolutely, but only inasmuch as it functions in accordance with certain notions, which we bring with us at birth, and that are common to all men of the world. Others do not admit of these notions and understand by natural light the evidence of objects that strike the mind, and that demand its consent.

3. By reason one sometimes means that very natural light by which the faculty that we refer to by this name is guided. It is in this way that one ordinarily understands the term when one is speaking of a proof, or of an objection taken from reason , and which one wants to distinguish in this way from proofs and objections grounded in divine or human authority. To the contrary, when one says that that reason is mistaken, or that it is subject to error, that it is blind, or that it is depraved, one means the faculty that we call reason ; because it is clear that such statements fit very well the faculty, and not at all the natural light.

4. By reason one can also mean the sequence of truths that the human mind can attain naturally, without being aided by the light of faith. The truths of reason are of two types: some are what one calls eternal truths , which are absolutely necessary, to the point that the opposite would imply contradiction. Such are truths of which the necessity is logical, metaphysical, or geometric. One could not overturn these truths without being led to absurdities. There are others that could be called positive truths , because they are the laws that it pleased God to give to nature, or because they depend on nature. We learn them either by experience, that is to say, a posteriori , or by reason , and a priori , that is to say by considerations of convenience that caused them to be preferred. This convenience also has its rules and its reasons , but it is the free choice of God, and not a geometric necessity, that the convenient be preferred. Thus one can say that physical necessity is founded on moral necessity, that is to say on the choice of the wise and worthy person, worthy of his wisdom, and that one as well as the other must be distinguished from geometric necessity. This physical necessity is what makes the natural order, and consists of the rules of movement and of a few other general laws that God established in creating this universe. The laws of nature are always subject to the disposal of the legislator, who can, if he so chooses, stop and suspend them, whereas eternal truths, such as those of Geometry, are not subject to any arbitrary law. Now it is those last truths that faith would never oppose. The truth can never be attacked by an invincible objection, because if this argument is grounded on principles or incontestable facts, and established by a sequence of eternal truths, the conclusion is certain and incontrovertible, and anything which is opposed to it must necessarily be false, or otherwise two conflicting propositions could be true at the same time. If the objection is not demonstrative, it is no more than a plausible argument, which does not have force against faith, since it must be agreed that the mysteries of religion are contrary to appearances. See the article Mysteries, in which it is proved against Bayle that faith is in agreement with reason , meaning that series of eternal truths that are absolutely necessary. It is now necessary to establish the precise boundaries that lie between faith and reason .

1. No proposition can be accepted as divine revelation if it contradicts what is known to us, either by immediate intuition, as in the case of self-evident propositions, or by obvious deductions of reason , as in demonstrations. It would be ridiculous to give preference to such revelations, because the evidence that causes us to adopt them cannot surpass the certainty of our intuitive or demonstrative knowledge, even if it may sometimes equal it; such a preference would mean reversing the principles and foundations of all knowledge and assent. There would no longer be any clear marks of truth and falsity, no measure of the credible and the incredible if doubtful propositions took precedence over self-evident propositions. It is therefore useless to press as articles of faith propositions contrary to the clear perception that we have of the rectitude of our ideas. Consequently, reason is the true competent judge in every thing of which we have a clear and distinct idea. Though revelation, in agreeing with reason , can confirm these judgments, it cannot nullify its decrees in such cases. Wherever we have a clear and evident judgment of reason , we cannot be forced to renounce it to embrace the contrary opinion under the pretext that it is a matter of faith. The reason for this is that we are men before we are Christians.

2. God, in granting us the light of reason , did not forsake the liberty to give us, as he saw fit, the assistance of revelation in matters where our natural faculties cannot reach. In cases where it pleases God to supply us with this extraordinary assistance, revelation must overcome all resistance of our reason ; this resistance being founded on nothing more than probable conjectures. As the mind is uncertain of the truth of what is not self-evident to it, and in such cases is carried along by probability, it is forced to give its assent to an account that it knows to come from he who can neither deceive nor be deceived. When the principles of reason do not make us see plainly that a proposition is true or false, manifest revelation can resolve the mind, standing as another principle of truth. In this case a proposition supported by revelation becomes a matter of faith, and is above reason . Reason not being able to rise above probability, faith guides the mind where reason falls short.

This far extends the influence of faith, without doing violence to reason , which is neither harmed nor harried, but aided and perfected by new lights emanating from the eternal source of all knowledge. Everything that is based on the jurisdiction of revelation must prevail upon our opinions, on our prejudices and on our interests, and has the right to demand of the mind its perfect assent. But such submission of our reason to faith may not reverse the limits of human knowledge, and does not shake the foundations of reason . Instead it leaves us the liberty to employ our faculties for the purposes for which they were given.

If one does not take care to distinguish between the different domains of faith and reason by establishing these boundaries, reason will have no jurisdiction over religion, and one will have no right to scoff at the extravagant opinions and ceremonies that one observes in the majority of world religions. Who does not see that this opening would mean the way to the most incensed fanaticism, and the most foolish superstitions! With such a principle, there would be nothing so absurd that it could not be believed. It follows then that religion, which is the pride of humanity and the most excellent advantage of our nature over beasts, is often the thing in the world in which men appear the most unreasonable.