As the logician must assume his premises, and, as a logician, cannot question their truth, so the physicist must assume a force in operation, and, as a physicist, cannot examine its genesis. The physical or the metaphysical method of inquiry is valid only so long as restricted to physical or metaphysical processes: a mixture of the two methods will give results satisfactory neither to science nor to philosophy. As logic furnishes no criterion by which to test the absolute truth of propositions, but deals wholly with conclusions drawn from given premises, so science furnishes no data by which to determine the absolute genesis of force, but restricts its enquiries to the phenomena resulting from a force given. For the student of physical science cause and effect is only the transference of a given and determined force from one material form to another. If this idea is to be traced further, it must be studied outside the limits of physics. This study belongs to metaphysics. Now, if physical science does not deal with the origin of the initial force, but assumes at the outset its presence, no more does it fall within its province to examine into the origin of the increments which give to physical forms that variety which renders science possible. Science deals with results, not antecedents; and after having determined results, it is not authorized to affirm that one species has produced another by evolution, or has produced it at all. If there are agreements between different organisms by which they are brought into relation, there are also differences by which they are discriminated, and these differences imply increments of force; and to assert that one organism has evolved another is to determine not merely the presence of this new increment, but also to determine its origin. Scientific investigation deals with phenomena which give evidence to the senses of a _transference_ of force from one form or from one manifestation to another. Transference is not increase--an effect can be no more than the evolution of what was potentially present in the cause; it cannot add to it. The origin of the force must be investigated according to intellectual laws.