Sentence:  Perhaps the reader will wonder why we have placed our " beings " on a sphere rather than on another closed surface. But this choice has its justification in the fact that, of all closed surfaces, the sphere is <WSD>unique</WSD> in possessing the property that all points on it are equivalent. 
Meaning of the word marked with <WSD> tags:  Being the only one of its kind; unequaled, unparalleled or unmatched.