A quantity-space is a set that has the property that a distance function is defined for any two elements in the set. In addition, the range of the distance function is a subclass of the class of scalar quantities.
(<=> (Quantity-Space ?S)
(And (Set ?S)
(Forall (?X1 ?X2)
(=> (And (Member ?X1 ?S) (Member ?X2 ?S))
(Exists (?D)
(And (= ?D (Distance ?X1 ?X2))
(Scalar-Quantity ?D)))))))
(Forall (?X1 ?X2)
(=> (And (Member ?X1 ?S) (Member ?X2 ?S))
(Exists (?D)
(And (= ?D (Distance ?X1 ?X2))
(Scalar-Quantity ?D)))))
(Set ?S)
(<=> (Quantity-Space ?S)
(And (Set ?S)
(Forall (?X1 ?X2)
(=> (And (Member ?X1 ?S) (Member ?X2 ?S))
(Exists (?D)
(And (= ?D (Distance ?X1 ?X2))
(Scalar-Quantity ?D)))))))
(=> (Point-In $X $Y) (Quantity-Space $Y))
(<=> (Point-In ?Pt ?Sp) (And (Quantity-Space ?Sp) (Member ?Pt ?Sp)))
(=> (= (Distance ?X1 ?X2) ?D)
(Exists (?Sp)
(And (Quantity-Space ?Sp)
(Point-In ?X1 ?Sp)
(Point-In ?X2 ?Sp))))