It may be said, that if it were self-evident that the mind cannot be caused to act, it would appear so to all men, and there could be no doubt on the subject; that a truth or proposition cannot be said to be self-evident, unless it carries irresistible conviction to every mind to which it is proposed. But this does not follow. Previous to the time of Galileo, it was universally believed by mankind, that if a body were set in motion, it would run down of itself; though it should meet with no resistance whatever in its progress. But that great philosopher, by reflecting on the nature of matter, very clearly saw, that if a body were put in motion, and met with no resistance, it would continue to move on in a right line forever. As matter is inert, so he saw that it could not put itself in motion; and if put in motion by the action of any thing upon it, he perceived with equal clearness that it could not check itself in its career. He perceived that it is just as impossible for passive, inert matter, to change its state from motion to rest, as it is for it to change its state from rest to motion. Thus, by simply reflecting upon the nature of matter, as that which cannot act, the mind of Galileo recognized it as a self-evident and unquestionable truth, that if a body be put in motion, and there is nothing to impede its career, it will move on in a right line forever. This great law of motion, first recognized by Galileo, and afterwards adopted by all other philosophers, is called the law of inertia; because its truth necessarily results from the fact, that matter is essentially inert, or cannot act. I am aware it has been contended by Mr. Whewell, in his Bridgewater Treatise, that the law of motion in question is not a necessary or self-evident truth; and the reason he assigns is, that if it were a truth of this nature, it would have been recognized and believed by all men before the time of Galileo. But this reason is not good. For if it did not appear self-evident to those philosophers who lived before Galileo, it was because they did not bestow sufficient reflection upon the subject, and not because it was not a self-evident truth. All men had seen bodies moving only in a resisting medium, amid counteracting influences; and having always seen them run down in such a medium, they very naturally concluded that a body put in motion would run down of itself. Yielding to an illusion of the senses, instead of rising above it by a sustained effort of reason and meditation, they supposed that the motion of a body would spend itself in the course of time, and so come to an end without any cause of its extinction. This is the reason why they did not see, what must have appeared to be a self-evident truth, if they had bestowed sufficient reflection upon the subject, instead of being swayed by an illusion of the senses.