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))))