I-1 is in the section of propositions that use only the first four postulates and, if you use all of Euclid’s assumptions, it does hold.

• I-1 is in the section of propositions that use only the first four postulates and, if you use all of Euclid’s assumptions, it does hold.

• I-1 does not hold in the spherical geometry model and the first four postulates hold here, too; therefore I-1 is independent of the first four postulates.

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