12. While it is necessary to keep the investigation of a process on its objective side, limited to objective conditions, and to express the result in objective terms, we must remember that this is an artifice; above all, we must remember that even within the objective limits our analyses are only provisional, and must be finally rectified by a restoration of all the elements we have provisionally set aside. Thus rectified, the objective interpretation of vital and mental phenomena has the incomparable advantage of simplifying research, keeping it fixed on physical processes, instead of being perturbed by suggestions of metaphysical processes. And as all physical investigation naturally tends to reduce itself to a mechanical investigation, because Mechanics is the science of motion, and all physical processes are motions, we may be asked, Why should not the mechanical point of view be the rational standing-point of the biologist? Our answer is, Because Mechanics concerns itself with abstract relations, and treats of products without reference to modes of production, i. e. with motions without reference to all the conditions on which they depend. Every physical change, if expressed in physical terms, is a change of position, and is determined by some preceding change of position. It is a movement having a certain velocity and direction, which velocity and direction are determined by the velocity and direction of a force (a pressure or a tension) compounded with the forces of resistance, i. e. counter-pressures. Clearly, the nature of the forces in operation must be taken into account; and it is this which the mechanical view disregards, the biological regards. The mechanical view is fixed on the ascertained adjustment of the parts, so that the working of the organism may be explained as if it were a machine, a movement here liberating a movement there. The biological view includes this adjustment of parts, but takes in also the conditions of molecular change in the parts on which the adjustment dynamically depends. Mechanical actions may be expressed as the enlargement or diminution of the angle of two levers; but chemical actions are not thus expressible; still less vital and mental actions.