motion can be thought (as he confesses) without having two motions in one time. Motion, in fact, is the synthesis of place and time, and cannot be comprehended except as their unity. The argument here quoted is only adduced by Mr. S. for the purpose of antithesis to other arguments on the other side as weak as itself. On page 241, Mr. Spencer deals with the question of the destructibility of matter: “The annihilation of matter is unthinkable for the same reason that the creation of matter is unthinkable.” (P. 54): “Matter in its ultimate nature is as absolutely incomprehensible as space and time.” The nature of matter is unthinkable, its creation or destructibility is unthinkable, and in this style of reasoning we can add that its _indestructibility_ is likewise unthinkable; in fact the argument concerning self-existence will apply here. (P. 31): “Self-existence necessarily means existence without a beginning; and to form a conception of self-existence is to form a conception of existence without a beginning. Now by no mental effort can we do this. To conceive existence through infinite past time, implies the conception of infinite past time, which is an impossibility.” Thus, too, we might argue in a strain identical; indestructibility implies existence through infinite future time, but by no mental effort can infinite time be conceived. And thus, too, we prove and disprove the persistence of force and motion. When occasion requires, the ever-convenient argument of “inconceivability” enters. It reminds one of Sir Wm. Hamilton’s “imbecility” upon which are based “sundry of the most important phenomena of intelligence,” among which he mentions the category of causality. If causality is founded upon imbecility, and all experience upon _it_, it follows that all empirical knowledge rests upon imbecility. On page 247, our author asserts that the first law of motion “is in our day being merged in the more general one, that motion, like matter, is indestructible.” It is interesting to observe that this so-called “First law of motion” rests on no better basis than very crude reflection. “When not influenced by external forces, a moving body will go on in a straight line with a uniform velocity,” is Spencer’s statement of it. This abstract, supposed law has necessitated much scaffolding in Natural Philosophy that is otherwise entirely unnecessary; it contradicts the idea of momentum, and is thus refuted: I. A body set in motion continues in motion after the impulse has ceased from without, for the reason that it retains momentum. II. Momentum is the product of weight by velocity, and weight is the attraction of the body in question to another body external to it. If all bodies external to the moving body were entirely removed, the latter would have no weight, and hence the product of weight by velocity would be zero.