The principles of indestructibility or conservation of matter and energy, flow from the preceding ones, and are exhibited in mental actions as well as in physical ones. Whilst the universal experience of mankind has not yet afforded us a single well verified instance of actual creation or destruction of matter or energy, it has supplied us with plenty of examples of apparent destruction and creation of each of them. {109} But scientific knowledge corrects the uncertain testimony of consciousness; whilst we see coal burn and be apparently destroyed, science proves to us that the elements composing it remain undiminished. We observe also that the heat of the fire dissipates and is apparently lost for ever; but science again proves that it is either stored up in the latent state, ready to be again liberated at a future time, or else converted into other forms of energy. A given atom of matter or a portion of energy, therefore, to the best of our knowledge, continues and persists for ever. As we cannot either create or destroy matter so also can we not create or annihilate energy, and this truth probably holds good with regard to mental as well as to physical and chemical power. Great changes of state in bodies (as in the combustion of wood &c.) have led us to erroneously think that the substances are destroyed; and great apparent differences of property, such as those of diamond and charcoal, have led us similarly to conclude that they are entirely distinct and independent of each other when they are not. As the cerebrum of man is composed of matter, and during excitement, its parts are active, we might confidently predict that its particles obey the First law of Motion, viz.: that a body in a state of rest or motion will continue in that state of rest or motion until some cause arise to prevent it. So it has been found that the action of the vital and mental forces have a degree of persistence, like the physical ones. It has been experimentally found that portions of {110} living bone transplanted to fleshy parts of animals where there was no bone, continued to grow for a time by a life of their own, and increased by formation of additional bone, like a crystal grows in its medium; but after a time they diminished and disappeared. In a similar manner we are all of us aware of the persistency of ideas, even in opposition to the will, after the cause of them has been removed. Sometimes we cannot retain an idea because of the persistence of others; and at other times we cannot get rid of one, for a similar reason. Our mental habits also have often very great persistence.