QuestionsCan we be certain of anything?Seeing with one's own eyes: the Earth moves
Can we be certain of anything?Yes, through reason

Seeing with one's own eyes: the Earth moves

Giordano Bruno 1548 · 1600The universe is infinite, filled with countless worlds.
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What it claims

The Earth moves, and one can be as certain of it as of anything else one holds certain, "not by authority but by living sense and reason".

Why

In natural philosophy authority proves nothing: one must judge with one's own eyes, by the senses and reason.

In short

"Seeing with one's own eyes: the Earth moves" is an argument by Giordano Bruno (The Ash Wednesday Supper, dial. I, III, IV and V (1584)) on the question "Can we be certain of anything?" (answer: yes, through reason). Its claim: The Earth moves, and one can be as certain of it as of anything else one holds certain, "not by authority but by living sense and reason". 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 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

The Ash Wednesday Supper, dial. I, III, IV and V (1584)
Valid with an implicit premise

In 1584 almost all scholars held the Earth to be motionless at the centre of the world, like Aristotle and Ptolemy. Copernicus had proposed the opposite in 1543, but an anonymous preface presented his book as a mere hypothesis for calculation. In The Ash Wednesday Supper, Bruno tells (with embellishments) of a dinner in London where he debates with two Oxford doctors. His thesis: we do not judge nature by authority, not even Copernicus's, nor by Scripture, but by the senses rightly interpreted and by reason. And so, he says, one can be as certain of the Earth's motion as of anything else. Teofilo reports the words of "the Nolan", that is, Bruno.

P1In natural philosophy, authority proves nothing: not Aristotle, not Ptolemy, not even Copernicus. One must see "with one's own eyes", that is, judge by living sense and reason.
P2Scripture does not settle these questions: when it speaks of natural things, it uses common language to teach conduct, not astronomy. There it must not serve as authority.
P3What our senses show (the sky turning, the Earth at rest) is explained just as well if it is the Earth that turns: this appearance comes from the Earth's rotation.
P4And reason gives positive grounds: Copernicus does not merely make a calculating hypothesis, he "demonstrates as a physicist" the Earth's motion; and in an infinite universe no body is the motionless centre of the whole.
P5 · implicit premiseWhen authority and Scripture are out of play, the appearances can be explained both ways, and reason gives grounds for one, reason can decide with certainty.
Therefore the Earth moves, and one can be as certain of it as of anything else one holds certain, "not by authority but by living sense and reason".

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 authority proves nothing, Scripture does not decide, the appearances can be explained both ways and there are grounds for the Earth's motion, but certainty is not reached: we remain in the probable.
Valid with P5. The first four premises clear away the obstacles and give reasons; P5 adds that this is enough to be certain. That is the weak point: one can grant P1 to P4 and conclude only that the Earth's motion is more probable, not certain. Bruno takes this step without hesitation. He admires Copernicus while reproaching him for being "more devoted to mathematics than to nature": for him, certainty comes from philosophical principles (the infinite universe, the soul of the stars) more than from calculation.
Analogy. On a train pulling out smoothly, we sometimes think it is the train next to us that is leaving. Our eyes are not enough to say who is moving: we must reason. Bruno says the same of the Earth and the sky. The train image is not his; his is the ship.

Objections

3

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

Aristotle (taken up by Ptolemy and the Oxford doctors) 350 BC · attacks premise 3

If the Earth turned very fast, a stone thrown straight up could not fall back to its starting point: the Earth would have moved beneath it, and the stone would land far to the west. Yet it falls back vertically. So the Earth is motionless (On the Heavens, book II; argument summarised by Smitho in the dialogue).

Bruno replies
Giordano Bruno 1584 · Teofilo, Bruno's spokesman: the ship example
With the Earth move all the things that are on the Earth. On a moving ship, a stone dropped from the top of the mast falls at the foot of the mast, whereas a stone dropped at the same spot by someone who is not on the ship is left behind. The only reason: the things fixed to the ship, or belonging to it, move with it, and the stone carries with it the force of the mover that moves with the ship.
The debate goes on (our summary) This is the argument Galileo developed in 1632 (Dialogue concerning the Two Chief World Systems), and it anticipates the principle of relativity. We do not know whether Galileo took it from Bruno, whom he never cites.
A
Andreas Osiander (anonymous preface to Copernicus's book) 1543 · attacks premise 4

The astronomer's hypotheses need not be true, nor even probable: it is enough that they yield calculations matching the observations. The Earth's motion is a tool for calculation, not a claim about reality. This is the preface "To the reader" placed unsigned at the head of Copernicus's book (text not collected).

Bruno replies
summary: their text is not yet in our corpus
Giordano Bruno 1584 · Teofilo, Bruno's spokesman
Copernicus affirmed the Earth's motion even in his letter to the Pope, and in his first book he does not only act as a mathematician who supposes, but also as a physicist who demonstrates the Earth's motion.
The debate goes on (our summary) Just before, Teofilo calls the preface's author an "ignorant and presumptuous ass". On Copernicus's intention Bruno is right: the preface was not his (Kepler revealed in 1609 that it was Osiander's). But Osiander's position, which separates calculation from truth, has its defenders in philosophy of science ("instrumentalism").
R
Robert Bellarmine 1615 · attacks premise 5

If there were a true demonstration that the Sun is at the centre and the Earth moves, one would have to reinterpret carefully the passages of Scripture that seem to say the opposite. But one must not believe such a demonstration exists until it is shown; meanwhile one must keep to the received sense (letter to Foscarini, 1615; text not collected). Bellarmine had taken part in Bruno's trial.

no reply
summary: their text is not yet in our corpus
No reply possible: objection raised in 1615, after the author's death (1600).
The debate goes on (our summary) Bruno had already answered about Scripture (P2), but not the demand for a demonstration. On that point Bellarmine puts his finger on the weakness of P5: in 1584 the decisive proofs were lacking. They came later: the aberration of light (Bradley, 1729), stellar parallax (Bessel, 1838), Foucault's pendulum (1851).