there is one common character of all such scientific operations which it is necessary to note. They are such as disclose relationships. A simple case is the operation by which length is defined by one object placed end upon end upon another object so many times. This type of operation, repeated under conditions them- selves defined by specified operations, not merely fixes the relation of two things to each other called their length, but defines ageneralized concept of length. This conception in connection with other operations, such as those which define mass and time, become instruments by means of which a multitude of relations between bodies can be established. Thus the conceptions which define units of measurement of space, time and motion become the intellectual instrumentalities by which all sorts of things with no qualitative similarity with one another can be compared and brought within the same system. To the original gross experience of things there is superadded another type of experience, the product of deliber- ate art, of which relations rather than qualities are the signifi- cant subject-matter. These connections are as much experi- enced as are the qualitatively diverse and irreducible objects of original natural experiences.