Converse of Euclid's Parallel Postulate
Euclid did not postulate the converse of his fifth postulate, which is one way to distinguish Euclidean geometry from elliptic geometry. The Elements contains the proof of an equivalent statement (Book I, Proposition 27): If a straight line falling on two straight lines make the alternate angles equal to one another, the straight lines will be parallel to one another. As De Morgan pointed out, this is logically equivalent to (Book I, Proposition 16). These results do not depend upon the fifth postulate, but they do require the second postulate which is violated in elliptic geometry.
Read more about this topic: Parallel Postulate
Famous quotes containing the words converse and/or parallel:
“It is said that desire is a product of the will, but the converse is in fact true: will is a product of desire.”
—Denis Diderot (1713–1784)
“We tend to be so bombarded with information, and we move so quickly, that there’s a tendency to treat everything on the surface level and process things quickly. This is antithetical to the kind of openness and perception you have to have to be receptive to poetry. ... poetry seems to exist in a parallel universe outside daily life in America.”
—Rita Dove (b. 1952)