QuestionsCan we be certain of anything?Reason is a correct scale, but mountains cannot be weighed on it
Can we be certain of anything?What escapes words

Reason is a correct scale, but mountains cannot be weighed on it

The refutation of philosophy

Ibn Khaldun 1332 · 1406Civilisations rise and fall with the strength of their solidarity.
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What it claims

The philosophers' metaphysics can give neither certainty about what lies beyond the senses, nor the happiness it promises. Reason is a correct scale, but it must not be used to weigh God's oneness or the other world.

Why

Our ideas come from what the senses perceive, so reason cannot establish anything certain about what lies beyond the senses.

In short

"Reason is a correct scale, but mountains cannot be weighed on it" is an argument by Ibn Khaldun (Muqaddima, I, 6, 30 ("A refutation of philosophy") and I, 6, 14) on the question "Can we be certain of anything?" (answer: what escapes words). Its claim: The philosophers' metaphysics can give neither certainty about what lies beyond the senses, nor the happiness it promises. Reason is a correct scale, but it must not be used to weigh God's oneness or the other world. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3). AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

Muqaddima, I, 6, 30 ("A refutation of philosophy") and I, 6, 14
Valid with an implicit premise

The philosophers of Islam, such as al-Farabi and Avicenna, disciples of Aristotle, thought reason can know everything, God and the soul included, and that supreme happiness is that knowledge. Ibn Khaldun, who had read their books closely, answers that our ideas come from what the senses perceive: about what lies beyond the senses, reason provides only conjectures. He does not reject reason for all that: it is "a correct scale", but a scale for gold, which cannot weigh mountains. He even keeps logic, which sharpens the mind. Four centuries later, Kant too would set limits to metaphysics, for other reasons.

P1Our general ideas are drawn by abstraction from the things our senses perceive, as a seal matches all its impressions in clay or wax.
P2Spiritual beings (God, the intelligences, the soul and the hereafter) are not perceived by the senses: the senses even form a veil between them and us.
P3 · implicit premiseReason has no path to things other than ideas drawn from what the senses perceive.
P4Happiness does not come from demonstration: the joy of the soul that perceives without an intermediary is obtained by removing the veil of the senses, as the Sufis do, and the happiness promised by religious law lies beyond all we can perceive.
Therefore the philosophers' metaphysics can give neither certainty about what lies beyond the senses, nor the happiness it promises. Reason is a correct scale, but it must not be used to weigh God's oneness or the other world.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a case where our ideas come from the senses, where spiritual beings are not perceived, and where reason can nonetheless demonstrate something about them by another path, for example by reasoning about what "to exist" means.
With P3, the argument is valid: if all our ideas come from the sensible (P1) and spiritual beings are not part of it (P2), reason can demonstrate nothing about them, provided it has no other path (P3). P4 settles the question of happiness. P3 is implicit: Ibn Khaldun only says that abstraction is possible only from the sensible. That is the point the philosophers contest: Avicenna proves God from the mere fact that something exists, without abstracting ideas from spiritual beings.
Analogy. A kitchen thermometer is accurate for measuring the temperature of a cake. If you try to measure the temperature of the Sun with it, it melts: that does not prove it was wrong, only that it is not made for that.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Avicenna 1030 · attacks premise 3

Reason does not need to abstract ideas from spiritual beings in order to know them. It is enough to start from the fact that something exists: whatever could fail to exist owes its existence to something else, and one necessarily goes back to a being that exists by itself. This is the "way of the truthful", which takes God as witness of everything, instead of going through creatures.

no reply
We have not yet found a direct reply by Ibn Khaldoun in the texts.
The debate goes on (our summary) We have not found a direct answer by Ibn Khaldun to this proof from existence. He only notes that Avicenna himself grants that the resurrection of bodies cannot be demonstrated by reason (Muqaddima, I, 6, 30), which does not touch the proof of God. He mainly reproaches the philosophers for stopping at the "first intellect" and for believing that reason encompasses all being. Avicenna maintains that God's existence and the soul's survival can be demonstrated. (Our summary.)
Averroes 1179 · attacks the conclusion

Religious law itself commands us to reflect on beings in order to know their Creator. The study of the books of the Ancients is therefore obligatory for those capable of it, and demonstration cannot contradict revelation, for truth does not oppose truth. Forbidding philosophy to capable minds is closing the door through which the law calls them to know God.

Ibn Khaldoun replies
Ibn Khaldoun
Such is the fruit of this craft (sharpening the mind), together with acquaintance with the doctrines and opinions of scholars; one also knows what harm it can do. Let whoever studies it beware, and take it up only after being saturated with religious law, Qur'an interpretation and jurisprudence: without the religious sciences, one can hardly remain safe from it.
The debate goes on (our summary) Ibn Khaldun had read Averroes' abridgements of Aristotle, but he does not quote the Decisive Treatise and does not answer it directly. The two men agree on one point: philosophy is not for everyone. They disagree on the essential: for Averroes, demonstration reaches truths about God; for Ibn Khaldun, it reaches only conjectures. (Our summary.)
M
Modern readers attacks the conclusion

He contradicts himself. When he judges history, he boasts of a "demonstrative method that admits of no doubts". Why would reason be sure when judging the past, and powerless when judging God? He leans on reason when it serves him, and on faith when it gets in his way.

Ibn Khaldoun replies
Ibn Khaldoun
This does not speak against the intellect and its perceptions: the intellect is a correct scale, whose indications are certain. But it should not be used to weigh God's oneness, the other world, the nature of prophecy, the divine attributes and anything beyond its level: that would be desiring the impossible.
The debate goes on (our summary) His position is consistent on paper: the historian's method deals with the observable world, metaphysics with what lies beyond it. His critics answer that it is faith, not reason, that draws the boundary here: one does not see what proof would show that reason stops exactly there. (Our summary.)