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Exploring selected reductio ad absurdum proofs in Book 1 of the Elements, weshow they include figures that are not constructed. It is squarely at odds withHartshorne's claim that "in Euclid's geometry, only those geometrical figuresexist that can be constructed with ruler and compass". We also present diagramsquestioning Manders' distinction between exact and co-exact attributes of adiagram, specifically, a model of semi-Euclidean geometry which satisfiesstraightness of lines and equality of angles and does not satisfy the parallelpostulate.

On diagrams accompanying reductio ad absurdum proofs in Euclid's Elements book I. Reviewing Hartshorne and Manders