XLIV

So much for instances which aid the senses, instances which are chiefly useful for the informative part of our subject. For information commences with the senses. But the whole business terminates in works, and as the former is the beginning, so the latter is the end of the matter. I will proceed therefore with the instances which are pre-eminently useful for the operative part. They are of two kinds, and seven in number, though I call them all by the general name of Practical Instances. In the operative part there are two defects and two corresponding prerogatives of instances. For operation either fails us or it overtasks us. The chief cause of failure in operation (especially after natures have been diligently investigated) is the ill determination and measurement of the forces and actions of bodies. Now the forces and actions of bodies are circumscribed and measured, either by distances of space, or by moments of time, or by concentration of quantity, or by predominance of virtue. And unless these four things have been well and carefully weighed we shall have sciences fair perhaps in theory, but in practice inefficient. The four instances which are useful in this point of view I class under one head as Mathematical Instances and Instances of Measurement.

Operation comes to overtask us, either through the admixture of useless matters, or through the multiplicity of instruments, or through the bulk of the material and of the bodies that may happen to be required for any particular work. Those instances therefore ought to be valued which either direct practice to the objects most useful to mankind; or which save instruments; or which spare material and provision. The three instances which serve us here I class together as Propitious or Benevolent Instances. These seven instances I will now discuss separately, and with them conclude that division of my subject which relates to the Prerogative or Rank of Instances.