There are various classes of motion which will fall under this definition, beside those already enumerated. An example of one of these classes suggests itself in the phenomenon of walking. An attentive observer will perceive various symptoms calculated to persuade him that every step he takes, during the longest journey, is the production of thought. Walking is, in all cases, originally a voluntary motion. In a child, when he learns to walk, in a rope-dancer, when he begins to practice that particular exercise, the distinct determination of mind, preceding each step, is sufficiently perceptible.
It may be absurd to say that a long series of motions can be the result of so many express volitions, when these supposed volitions leave no trace in the memory. But it is not unreasonable to believe that a species of motion which began in express design may, though it ceases to be the subject of conscious attention, owe its continuance to a continued series of thoughts flowing in that direction, and that, if life were taken away, material impulse would not carry on the exercise for a moment. We actually find that, when our thoughts in a train are more than commonly earnest, our pace slackens, and sometimes our going forward is wholly suspended, particularly in any less common species of walking, such as that of descending a flight of stairs. In ascending the case is still more difficult, and accordingly we are accustomed wholly to suspend the regular progress of reflection during that operation.
Another class of motions of a still subtler nature are the regular motions of the animal economy, such as the circulation of the blood, and the pulsation of the heart. Are thought and perception the medium of these motions? We have the same argument here as in the former instances, conjunction of event. When thought begins, these motions also begin; and, when it ceases, they are at an end. They are therefore either the cause or effect of percipiency, or mind; but we shall be inclined to embrace the latter side of this dilemma when we recollect that we are probably acquainted with many instances in which thought is the immediate cause of motions, which scarcely yield in subtlety to these; but that, as to the origin of the faculty of thought, we are wholly uninformed. Add to this that there are probably no motions of the animal economy which we do not find it in the power of volition, and still more of our involuntary sensations, to hasten or retard.