It will be seen presently that Dr. Tyndall admits that the interference of personal volition in the economy of nature is not forbidden by the law of the conservation of energy. The point we now insist upon is that he has not succeeded in showing that this principle is an absolute and universal law of nature. We have already seen that it is limited and conditioned by the law of the dissipation of energy, and that in reality "it is merely a kind of _movable equilibrium_ between supply and destruction."[469] By no experimental evidence has it been shown that it holds true in the realm of vital dynamics and psycho-dynamics. There are able scientific men who question its absolute certainty even in the realm of physics. Professor Brooke says that "the amount of energy in the world is unchanged, the sum of the actual or kinetic and potential energies being a constant quantity has been by some writers overstrained. It may be taken as a postulate, and is probably true; but it is a proposition equally incapable of proof and of disproof."[470] To the same effect are the words of Sir John Herschel,[471] and still more recently of Professor Jevons.[472] "Nature," says Dr. Cohn, of Breslau, "is an equation with very many unknown quantities. It is the work of natural science to determine the value of these quantities. Some believe it never will be possible to solve the equation, since in it factors occur which can not be determined." Until this is done, it is simply presumptuous for Dr. Tyndall to pretend to know all the antecedents which determine the complex phenomena of nature, and dogmatically to affirm that "no new agency is created," and no "interference of Divine agency" can be permitted. "Our knowledge of things is finite, while our ignorance is infinite; and we must consequently regard all known lines of causation as being liable to be cut through by unknown ones." For aught we know to the contrary one of the unknown factors in the equation may be "personal volition," may be the ceaseless energy of the Divine Will sustaining and carrying nature forward through successive stages toward a predestinated goal. The foremost physicists do not deny that there may possibly be forms of energy which are neither potential nor kinetic.[473] We venture to assert with Prof. Challis that will, or personal energy, is neither the one nor the other, but the source of both. Mind is the originator, and matter is the recipient of force.[474]