Euclid — "Magnitudes which can be made to coincide are equal."
Magnitudes which can be made to coincide are equal.
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More from Euclid
"If a straight line be cut into two equal parts and also into two unequal parts, the rectangle contained by the unequal parts together with the square on the line between the points of section is equal…"
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Verification
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Understanding this quote
What it means
Two things are equal if they can be perfectly overlaid on each other — no gaps, no overflow, complete overlap. Equality isn't declared abstractly; it's proven by physical or logical superposition. This is a concrete, testable definition of sameness, grounding abstract mathematics in observable reality rather than assumption or authority.
Relevance to Euclid
Euclid built geometry on rigorous axioms and definitions in his Elements, refusing to assume what could be proven. This principle reflects his insistence on demonstration over declaration. As a systematizer who compiled and formalized Greek mathematical knowledge around 300 BCE, he grounded every theorem in foundational truths exactly like this one.
The era
In ancient Greece, mathematics was transitioning from practical measurement — land surveying, architecture, astronomy — to abstract logical proof. Euclid worked in Alexandria under Ptolemy I, where the great Library fostered intellectual rigor. This definition answered philosophical disputes about what 'equality' means, a live debate among Platonic and Aristotelian thinkers of his era.
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