We sum up what has been said in the preceding paragraphs on the uniformity of nature in the following words: We admit that the uniformity of the _constitution_ of nature is a self-evident and necessary truth. We admit also that, so far as our experience extends, the uniformity of the _course_ of nature must be admitted as a scientific truth, for to deny this would be to deny the possibility of all science, inasmuch as all science is _prevision_. But at the same time we maintain that the conclusions of scientific inference must always be of a hypothetical and purely provisional character, because it is impossible for us to judge with absolute certainty how far our experience gives us adequate information of the universe as a whole, and of all the forces and phenomena which can have place therein.[475] The conservation of energy, for example, is a very probable hypothesis which accords satisfactorily with the experiments of scientific men during a few years past, but it would be a gross misconception of the nature of scientific inference to suppose that it is certain in the same sense that a proposition in geometry is certain, or that any fact of immediate consciousness is certain.[476] Admitting the principle of the uniformity of nature as a hypothetical inference from a limited experience, we advance to the main position of Dr. Tyndall, namely, _that personal volition can not mingle in or interfere with the procession of phenomena in nature_. Dr. Tyndall admits the reality of "personal volition." We have not discovered in his writings any indications of the tendency manifested by some of his scientific associates to reduce volition to a form of physical energy. He grants "the power of free-will in man,"[477] but he seems unwilling to admit that free-will can exert any controlling, modifying, or determining influence on the procession of phenomena. "Assuming the efficacy of prayer to produce changes in external nature, it necessarily follows that natural laws are more or less at the mercy of man's volition, and no conclusion founded on the assumed permanence of those laws would be worthy of confidence."[478] But are not natural laws more or less subject to man's volition? Does he not act _upon_ the chain of cause and effect in nature, and _alter_ the procession of phenomena on earth? Certainly he can and does control and direct the forces of nature. He can so collocate and adjust the properties and forces of matter as to accomplish the purposes of his intelligence, and bring about _new_ results which would not otherwise have been produced. That man has materially modified the physical geography of the globe can not be denied. He has altered the climatal condition of whole tracts of country, and changed the physiognomy of the globe. The rain-fall has been changed by the felling of timber or the planting of trees.[479] He has extended or circumscribed the geographical boundaries of plants and animals.