QuestionsCan we be certain of anything?By convention sweet, in reality atoms and the void
Can we be certain of anything?We must doubt

By convention sweet, in reality atoms and the void

The distinction between convention and reality

Democritus c. 460 BC · c. 360 BCNothing exists but atoms and the void.
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What it claims

Sweet, bitter, hot, cold and colour exist by convention; in reality there are only atoms and the void. The senses do not give us the truth of things.

Why

What we sense changes with the state of our body, so it cannot tell what a thing is: in reality there are only atoms and void.

In short

"By convention sweet, in reality atoms and the void" is an argument by Democritus (Fragments B 9, B 11 (Sextus Empiricus) · Diogenes Laertius IX, 44-45) on the question "Can we be certain of anything?" (answer: we must doubt). Its claim: Sweet, bitter, hot, cold and colour exist by convention; in reality there are only atoms and the void. The senses do not give us the truth of things. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P2). AskPhilosophy presents 2 objections, 0 of them with a documented reply, and the original text behind each premise, checked word for word.

Fragments B 9, B 11 (Sextus Empiricus) · Diogenes Laertius IX, 44-45
Valid with an implicit premise

Democritus left a famous formula: by convention sweet, bitter, hot, cold, colour; in reality atoms and the void. It comes to us from Sextus Empiricus (c. 200 AD), who quotes it for his own theses, and Diogenes Laertius confirms it in his summary. The reasons we give here are a reconstruction: Democritus did not write them in this form, and only pieces remain. Filing: we put it under "We must doubt", because its lesson concerns the senses, which do not give the truth of things. This is an approximate filing: Democritus is not a total sceptic, since he believes in a "genuine" knowledge by thought (B 11) and in a complete theory of nature. That fragment would point rather to the camp "Yes, through reason"; but others (B 117: "in truth we know nothing") and Aristotle ("either there is no truth or we cannot discover it") pull him towards doubt. None of our camps says exactly "the senses deceive, thought goes further, but without certainty".

P1What we grasp changes with the state of our body and with what reaches it. (Democritus, B 9, quoted by Sextus)
P2 · implicit premiseWhat changes with the state of the one who senses, and is given only by the obscure knowledge of the senses, does not tell what the thing is in truth: it is a matter of convention.
P3There are two forms of knowledge: one obscure (sight, hearing, smell, taste, touch), the other genuine, which is distinct from it. (Democritus, B 11, quoted by Sextus)
P4The first principles of the universe are atoms and the void; everything else is merely thought to exist. (Diogenes Laertius, IX, 44, summary of his doctrine)
Therefore sweet, bitter, hot, cold and colour exist by convention; in reality there are only atoms and the void. The senses do not give us the truth of things.

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 P2, the computer finds a case where qualities change with the body, where the senses are obscure, and where sweet and bitter are nonetheless real properties of things (which would simply vary with conditions): the conclusion falls.
The argument is valid if P2 is added. With P1 and P3 we know that sensory qualities vary and come from obscure knowledge; P2 says this makes them conventions, with no truth of their own. P4 supplies the second half of the conclusion. P2 is written nowhere in our fragments: it is the implicit premise we add out of charity, and it is where one can attack. Note: P4 is the thesis that the formula of B 9 already states; the argument mainly shows why qualities are not real, not how we know atoms exist.
Analogy. The same bucket of lukewarm water feels hot to a cold hand and cold to a hot hand. We do not say that the water is both hot and cold: we say that "hot" and "cold" tell us something about our hands. Democritus does the same for sweet, bitter and colour, and looks behind them for a reality that does not change.

Objections

2

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

Aristotle (who ranks Democritus with those who confuse thinking and sensing) 335 BC · attacks the conclusion

Aristotle reports that Democritus says "either there is no truth or we cannot discover it". He explains this position by a starting error: one supposes that thinking is sensing, and that sensing is a change in the body. The impression of the senses would then be necessarily true, and since impressions contradict one another, nothing true could be known. For Aristotle, this is the weak point of the whole family of thought Democritus belongs to.

no reply
No reply possible: objection raised in -335, after the author's death (-360).
The debate goes on (our summary) Aristotle's diagnosis fits B 11 badly: there Democritus separates the obscure knowledge of the senses from the genuine knowledge of thought, which does not confuse thinking and sensing. Yet Aristotle writes elsewhere (On the Soul I 2) that Democritus takes the true to be what appears. The two witnesses contradict one another, and scholars disagree. Democritus had raised the objection himself. In a fragment quoted by Galen (B 125), outside our corpus, the senses reproach the mind for taking its proofs from them and then overthrowing them, and tell it that its victory would be its fall. Thought that rejects the senses depends on them: we do not know how Democritus answered.
Aristotle (on the origin of the atoms' motion) 340 BC · attacks premise 4

Leucippus and Democritus make atoms and the void the elements of everything. But where does motion come from, and how do beings have it? Aristotle writes that they neglected this question, like the others. A theory explaining everything by atoms and the void leaves unanswered what sets the atoms in motion.

no reply
No reply possible: objection raised in -340, after the author's death (-360).
The debate goes on (our summary) Diogenes Laertius, six centuries later, reports that the atoms are carried in a vortex through the universe and that Democritus calls this vortex necessity (IX, 44-45). It looks like an answer, but no fragment of Democritus sets it out, and Aristotle does not cite it here. So we do not know whether Democritus really dealt with the question.