147–148
Socrates
SOC. And did you find such a name?
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Theaetetus
THEAET. I think we did. But see if you agree.
Theaetetus
THEAET. We divided all number into two classes. The one, the numbers which can be formed by multiplying equal factors, we represented by the shape of the square and called square or equilateral numbers.
Theaetetus
THEAET. The numbers between these, such as three and five and all numbers which cannot be formed by multiplying equal factors, but only by multiplying a greater by a less or a less by a greater, and are therefore always contained in unequal sides, we represented by the shape of the oblong rectangle and called oblong numbers.
Socrates
SOC. Very good; and what next?
Theaetetus
THEAET. All the lines which form the four sides of the equilateral or square numbers we called lengths, and those which form the oblong numbers we called surds, because they are not commensurable with the others in length, but only in the areas of the planes which they have the power to form. And similarly in the case of solids. That is, cubes and cube roots.
Socrates
SOC. Most excellent, my boys! I think Theodorus will not be found liable to an action for false witness.
Theaetetus
THEAET. But really, Socrates, I cannot answer that question of yours about knowledge, as we answered the question about length and square roots. And yet you seem to me to want something of that kind. So Theodorus appears to be a false witness after all.
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