QuestionsCan we be certain of anything?The principle of all things is water
Can we be certain of anything?Through experience, with care

The principle of all things is water

Thales c. 625 BC · c. 545 BCThe principle of all things is water.
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What it claims

Water is the principle of all things.

Why

The food, heat and seeds of all beings are moist, so water seems to be the origin of everything.

In short

"The principle of all things is water" is an argument by Thales (Aristotle, Metaphysics A 3 (983b), about Thales) on the question "Can we be certain of anything?" (answer: through experience, with care). Its claim: Water is the principle of all things. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P5). AskPhilosophy presents 2 objections, 0 of them with a documented reply, and the original text behind each premise, checked word for word.

Aristotle, Metaphysics A 3 (983b), about Thales
Valid with an implicit premise

Thales left nothing in writing. This argument is the one Aristotle attributes to him, about two centuries after his death. Aristotle writes that Thales "says the permanent entity is water", then that Thales came to this idea "presumably" (in Greek: "perhaps") from a few observations. The reasons below are therefore the ones Aristotle supposes, not reasons Thales gave. We present them in their strongest form. A "principle" (arche) here means what things come from. Filing: none of our questions asks "what is the world made of?". We file the argument under "Through experience", because the reasoning starts from observations (food, heat, seeds). It is not a position Thales took on certainty.

P1The food of all beings is moist, and heat itself comes from the moist and lives by it. (A reason supposed by Aristotle, with "perhaps".)
P2The seeds of all things are moist by nature. (Another reason supposed by Aristotle.)
P3What a thing comes from is its principle.
P4Water is the principle of the nature of the moist.
P5 · implicit premiseWhat is true of food, heat and seeds is true of all things: beings that feed, warm and sprout stand for the whole world.
Therefore water is the principle of all 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 P5, the computer finds a case where food, heat and seeds are moist, where water is the origin of the moist, and where nevertheless water is not the principle of all things (stones and stars would come from something else).
With P5 the conclusion follows. Without it, nothing connects the observations (food, heat, seeds) to "all things": a stone or a star neither feeds nor sprouts. P5 is written nowhere: it is our implicit premise, added by charity, and it is the weak point of the argument. So one can attack it without denying the observations.
Analogy. In my kitchen, the vegetables, the dishes and my glass have all been in contact with water. That does not prove the oven, the tiles and the light come from water. We need a reason to believe these examples stand for everything else.

Objections

2

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

Aristotle (who reports the idea) 340 BC · attacks the conclusion

Even if everything comes from a single matter, this does not explain change. Wood does not make the bed, bronze does not make the statue: something else is needed to cause the change. A single matter, like water, leaves unanswered the question "who or what sets things in motion?".

no reply
No reply possible: objection raised in -340, after the author's death (-545).
The debate goes on (our summary) Thales cannot answer: he died about two centuries before this passage and wrote nothing. A defender can note that Aristotle himself writes elsewhere (On the Soul I 2) that Thales would have seen a soul in the magnet, because it moves iron, and that "all things are full of gods": water would then not act alone. But these are other reports by Aristotle, with "he seems" and "perhaps": we do not know whether Thales made that link.
Anaximenes and Diogenes of Apollonia (according to Aristotle) 340 BC · attacks the conclusion

Other thinkers after Thales gave another principle. Aristotle writes that Anaximenes and Diogenes put air before water, as the principle of all simple bodies. Why water rather than air, or fire (Heraclitus, Hippasus)? Aristotle's passage does not give their reasons: we have only the rival thesis here, not its argument.

no reply
We have not yet found a direct reply by Thalès in the texts.
The debate goes on (our summary) Thales wrote nothing, and we have no reply from him. A defender of water could appeal to the observations in P1 and P2 (food, heat, seeds), which the defenders of air would also have to explain. This remains our reconstruction: Anaximenes' reasons are not yet in our corpus.