QuestionsCan we be certain of anything?Every sensation is true
Can we be certain of anything?Through experience, with care

Every sensation is true

Sensations as the criterion of truth in Epicurus

Épicure 341 BC · 270 BCHe rejects the fate of the natural philosophers: some things happen by necessity, others by chance, others through our own agency.
See profile
What it claims

No sensation is false and we must not reject them: error is corrected by checking our opinions against our sensations, not by doubting them.

Why

A sensation only records what affects it; error comes from the judgment we add, and without sensations we can judge nothing.

In short

"Every sensation is true" is an argument by Épicure (Letter to Herodotus, 38 and 50-52; Principal Doctrines 23-24) on the question "Can we be certain of anything?" (answer: through experience, with care). Its claim: No sensation is false and we must not reject them: error is corrected by checking our opinions against our sensations, not by doubting them. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P4). AskPhilosophy presents 3 objections, 3 of them with a documented reply, and the original text behind each premise, checked word for word.

Letter to Herodotus, 38 and 50-52; Principal Doctrines 23-24
Valid with an implicit premise

A stick in water looks broken. A dream looks real. Must we conclude that the senses deceive us? Epicurus says no. A sensation only receives the effect of what causes it: it adds nothing, takes nothing away, so it is faithful. Error appears when we add a judgment ("it is a broken stick") that experience does not confirm or contradicts. For Epicurus, rejecting the senses is impossible anyway: anything we can call true or false is checked by going back to them.

P1A sensation only records the effect of what causes it: it adds nothing and takes nothing away. So it could be false only through an error coming from itself alone, with no added judgment.
P2Error always comes from a judgment added to the sensation (an opinion that experience does not confirm or contradicts).
P3If we reject all sensations, we have nothing left to refer to in order to judge, even the sensations we call false.
P4 · implicit premiseWe do have a way of judging what is true or false, and we must keep it.
Therefore no sensation is false and we must not reject them: error is corrected by checking our opinions against our sensations, not by doubting them.

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 P4, the computer finds a case where all sensations are rejected without destroying any standard, because there was none: the position of a sceptic who gives up judging.
The argument is valid with the hidden premise P4. P1 and P2 show that a sensation cannot be false: an error in the sensation alone comes from no added judgment, yet every error comes from an added judgment. P3 and P4 show that we cannot reject them either. The objections target P1 and P2: they say the sense itself can deceive us.
Analogy. A thermometer reading 5 degrees is not wrong: it faithfully shows the heat it receives. If I say "it is cold for everyone", it is my judgment that may be false. Epicurus treats the senses like that thermometer.

Objections

3

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

Plato (Theaetetus) 369 BC · attacks the conclusion

Socrates shows that sensation (seeing, hearing, feeling cold) reaches neither the being of things nor truth. So knowledge is not in impressions of sense, but in the reasoning we do about them. Saying every sensation is true does not make the senses the source of knowledge.

Épicure replies
E
Epicurus (the Canon, transmitted by Diogenes Laertius) 300 BC
Epicurus does not answer Plato by name. But he says reasoning cannot judge the senses, because all reasoning draws its notions from sensations. For him, "true" means "faithful to what affects it", not "complete knowledge": reasoning comes after and depends on the senses.
Where the debate stands (our summary)Our summary: a deep disagreement about the starting point of knowledge. For Plato, truth requires the intellect that grasps being; for Epicurus, it starts with sensations. The debate between rationalists and empiricists continues in other forms.
René Descartes (Meditations I) 1641 · attacks premise 2

I have learned everything I held most true from the senses or through the senses, yet I have found that they sometimes mislead. It is prudent never to trust entirely those who have deceived us even once. So the senses themselves can fail, not only our judgment.

a defender replies
see the argument
No reply possible: objection raised in 1641, after the author's death (-270).
T
The Epicureans (according to Diogenes Laertius, X, 34) defender of the argument · 300 BC
Epicureans answer this kind of case with the example of the tower. From afar it looks round: the sensation is faithful to what happens to the eye. The mistake is to claim "it is round" without waiting. One must "wait and get close to the tower" and see what it looks like close up. The judgment awaits confirmation; the sensation itself was not false.
Where the debate stands (our summary)Our summary: Descartes and Epicurus agree that the senses alone do not suffice for knowledge. They differ on what counts as "deception". For Epicurus, deception comes from the judgment; for Descartes, cases of illusion (dream, phantom limb) show that even sensation can mislead. The debate is about what "false" means.
Sextus Empiricus (the sceptical modes) 190 · attacks the conclusion

The same object appears in opposite ways depending on animals, persons and their condition (honey seems sweet to one, bitter to a sick person). These sensations cannot all be true of the object, and nothing allows us to choose between them. So we must suspend judgment on the nature of the object.

Épicure replies
No reply possible: objection raised in 190, after the author's death (-270).
E
Epicurus (the Canon, transmitted by Diogenes Laertius) 300 BC · Epicurus (earlier text)
Epicurus says one sensation cannot refute another, neither of the same kind (they have equal value) nor of another kind (they do not judge the same things). Sweet honey and bitter honey are two true sensations, each of what affects a particular organism. The error would be to say "honey is bitter in itself".
Where the debate stands (our summary)Our summary: Epicurus' answer saves sensations by tying them to the one who senses. Sceptics reply that then sensations no longer tell us anything about the object itself. Epicurus adds that we find out what the object is by comparisons and confirmations. An open debate.