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The Flames Test
David Hume
An Enquiry Concerning Human Understanding

The author issues a provocative command to discard any books that contain neither mathematical reasoning nor experimental facts, labeling them as mere illusion.

...tion in reason, so far as it is supported by experience. But its best and most solid foundation is faith and divine revelation. Morals and criticism are not so properly objects of the understanding as of taste and sentiment. Beauty, whether moral or natural, is felt, more properly than perceived. Or if we reason concerning it, and endeavour to fix its standard, we regard a new fact, to wit, the general tastes of mankind, or some such fact, which may be the object of reasoning and enquiry.
When we run over libraries, persuaded of these principles, what havoc must we make? If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.
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Blaise Pascal

Pensees

Heart's First Principles

Pascal challenges the strict empiricist dichotomy by arguing that we have an intuitive knowledge of first principles (such as space, time, number) through the 'heart,' which is neither abstract reasoning nor experimental reasoning. This suggests that Hume's rule would mistakenly dismiss fundamental truths that are known with certainty but not through the prescribed avenues.

...Is it by reason that you love yourself? 278 It is the heart which experiences God, and not the reason. This, then, is faith: God felt by the heart, not by the reason. 279 Faith is a gift of God; do not believe that we said it was a gift of reasoning. Other religions do not say this of their faith. They only gave reasoning in order to arrive at it, and yet it does not bring them to it. 280 The knowledge of God is very far from the love of Him. 281 Heart, instinct, principles. 282
We know truth, not only by the reason, but also by the heart, and it is in this last way that we know first principles; and reason, which has no part in it, tries in vain to impugn them. The sceptics, who have only this for their object, labour to no purpose. We know that we do not dream, and however impossible it is for us to prove it by reason, this inability demonstrates only the weakness of our reason, but not, as they affirm, the uncertainty of all our knowledge. For the knowledge of first principles, as space, time, motion, number, is as sure as any of those which we get from reasoning....
(We have intuitive knowledge of the tri-dimensional nature of space, and of the infinity of number, and reason then shows that there are no two square numbers one of which is double of the other. Principles are intuited, propositions are inferred, all with certainty, though in different ways.) And it is as useless and absurd for reason to demand from the heart proofs of her first principles, before admitting them, as it would be for the heart to demand from reason an intuition of all demonstrat...

Blaise Pascal

Pensees

Reason's Necessary Limits

Pascal proposes an alternative approach to reason and faith: sometimes it is rational to disavow reason in order to accommodate the mysterious elements of religion. This stands in opposition to Hume's insistence that only reasoning concerning quantity/number or matter of fact is legitimate, offering a different rule for engaging with theological texts.

...ing everything, from want of knowing where to submit; or by submitting in everything, from want of knowing where they must judge. 269 Submission is the use of reason in which consists true Christianity. 270 St. Augustine.[104]--Reason would never submit, if it did not judge that there are some occasions on which it ought to submit. It is then right for it to submit, when it judges that it ought to submit. 271 Wisdom sends us to childhood. Nisi efficiamini sicut parvuli.[105] 272
There is nothing so conformable to reason as this disavowal of reason. 273 If we submit everything to reason, our religion will have no mysterious and supernatural element. If we offend the principles of reason, our religion will be absurd and ridiculous.
274 All our reasoning reduces itself to yielding to feeling. But fancy is like, though contrary to feeling, so that we cannot distinguish between these contraries. One person says that my feeling is fancy, another that his fancy is feeling. We should have a rule. Reason offers itself; but it is pliable in every sense; and thus there is no rule. 275 Men often take their imagination for their heart; and they believe they are converted as soon as they think of being converted. 276 M. de...

David Hume

An Enquiry Concerning Human Understanding

Metaphysics as Antidote

This excerpt explains the deeper rationale behind Hume's prescription: human understanding is not equipped to handle remote and abstruse subjects, and the only way to free learning from such futile inquiries is to conduct a precise analysis of our cognitive capacities. This grounds the rule in a theory of the limits of human knowledge.

...reasonably have place in the sciences; since, however unsuccessful former attempts may have proved, there is still room to hope, that the industry, good fortune, or improved sagacity of succeeding generations may reach discoveries unknown to former ages. Each adventurous genius will still leap at the arduous prize, and find himself stimulated, rather that discouraged, by the failures of his predecessors; while he hopes that the glory of achieving so hard an adventure is reserved for him alone.
The only method of freeing learning, at once, from these abstruse questions, is to enquire seriously into the nature of human understanding, and show, from an exact analysis of its powers and capacity, that it is by no means fitted for such remote and abstruse subjects. We must submit to this fatigue, in order to live at ease ever after: And must cultivate true metaphysics with some care, in order to destroy the false and adulterate. Indolence, which, to some persons, affords a safeguard against this deceitful philosophy, is, with others, overbalanced by curiosity; and despair, which, at some...
8. Besides this advantage of rejecting, after deliberate enquiry, the most uncertain and disagreeable part of learning, there are many positive advantages, which result from an accurate scrutiny into the powers and faculties of human nature. It is remarkable concerning the operations of the mind, that, though most intimately present to us, yet, whenever they become the object of reflexion, they seem involved in obscurity; nor can the eye readily find those lines and boundaries, which discrimin...

David Hume

An Enquiry Concerning Human Understanding

Ideas from Impressions

This passage provides a concrete method for implementing Hume's empirical criterion: when confronted with any idea, one should trace it back to the impression from which it is copied. If no such impression can be found, the idea lacks empirical content and should be dismissed. This is a practical tool for evaluating the meaningfulness of concepts in any volume.

...rmer, require superior care and capacity to be surmounted. 49. There are no ideas, which occur in metaphysics, more obscure and uncertain, than those of power, force, energy or necessary connexion, of which it is every moment necessary for us to treat in all our disquisitions. We shall, therefore, endeavour, in this section, to fix, if possible, the precise meaning of these terms, and thereby remove some part of that obscurity, which is so much complained of in this species of philosophy.
It seems a proposition, which will not admit of much dispute, that all our ideas are nothing but copies of our impressions, or, in other words, that it is impossible for us to think of any thing, which we have not antecedently felt, either by our external or internal senses. I have endeavoured to explain and prove this proposition, and have expressed my hopes, that, by a proper application of it, men may reach a greater clearness and precision in philosophical reasonings, than what they have hitherto been able to attain. Complex ideas may, perhaps, be well known by definition, which is not...
These impressions are all strong and sensible. They admit not of ambiguity. They are not only placed in a full light themselves, but may throw light on their correspondent ideas, which lie in obscurity. And by this means, we may, perhaps, attain a new microscope or species of optics, by which, in the moral sciences, the most minute, and most simple ideas may be so enlarged as to fall readily under our apprehension, and be equally known with the grossest and most sensible ideas, that can be the...