It has gradually dawned upon me that the reason why Philosophers who are well acquainted with Physical or Dynamical Science are apt to fall into the error of supposing that mental and vital interference with the material world is impossible, in spite of their clamorous experience to the contrary (or else, on the strength of that experience, to conceive that there is something the matter with the formulation of physical and dynamical laws), is because all such interference is naturally and necessarily excluded from scientific methods and treatises. In pure Mechanics, "force" is treated as a function of configuration and momentum: the positions, the velocities, and the accelerations of a conservative system depend solely on each other, on initial conditions, and on mass; or, if we choose so to express it, the co-ordinates, the momenta, and the kinetic energies, of the parts of any dynamical system whatever, are all functions of time and of each other, and of nothing else. In other words, we have to deal, in this mode of regarding things, with a definite and completely determinate world, to which prediction may confidently be applied. But this determinateness is got by refusing to contemplate anything outside a certain scheme: it is an internal truth within the assigned boundaries, and is quite consistent with psychical interference and indeterminateness, as soon as those boundaries are ignored; determinateness is not part of the _essence_ of dynamical doctrine, it is arrived at by the tacit assumption that no undynamical or hyperdynamical agencies exist: in short, by that process of abstraction which is invariably necessary for simplicity, and indeed for possibility, of methodical human treatment. Everyone engaged in scientific research is aware that if exuberant charwomen, or intelligent but mischievous students (who for the moment may be taken to represent life and mind respectively) are admitted into a laboratory and given full scope for their activities, the subsequent scientific results—though still, no doubt, in some strained sense, concordant with law and order—are apt to be too complicated for investigation; wherefore there is usually an endeavour to exclude these incalculable influences, and to make a tacit assumption that they have not been let in. There is a similar tacit assumption in treatises on Physics and Chemistry; viz., that the laws of automatic nature shall be allowed unrestricted and unaided play, that nothing shall intervene in any operation from start to finish save mechanical sequent and antecedent,—that it is permissible in fact to exercise abstraction, as usual, to the exclusion of agents not necessarily connected with the problem, and not contemplated by the equations.