says Prof. Challis, "than pressure by contact of one body with another. This hypothesis is made on the principle of admitting no fundamental ideas that are not referable to sensation and experience. It is true that we see bodies obeying the influence of an external force, as when a body descends toward the earth by the action of gravity; and, so far as the sense of sight informs us, we do not in such cases perceive either the contact or the pressure of another body. But we have also the sense of touch or of pressure by contact—for instance, of the hand with another body—and we feel in ourselves the power of causing motion by such pressure. The consciousness of this power and the sense of touch give a distinct idea, such as all the world understands and acts upon, as to how a body may be moved; and the rule of philosophy which makes personal sensation and experience the basis of scientific knowledge, as they are the basis of the knowledge that regulates the common transactions of life, forbids recognizing any other mode of moving a body than this. When, therefore, a body is caused to move without apparent contact and pressure of another body, it must still be concluded that the pressing body, although invisible, exists, unless we are prepared to admit that there are physical operations which are and ever will be incomprehensible by us. This admission is incompatible with the principles of the philosophy I am advocating, which assume that the information of the senses is adequate, with the aid of mathematical reasoning, to explain phenomena of all kinds. . . . All physical force being pressure, there must be a medium by which the pressure is exerted."
It is not my purpose, of course, to question the Newtonian doctrine of gravitation, or to urge the adoption of the views of Prof. Challis and others, who seek to show that what seems to be attraction is in reality a propulsion of solid bodies in immediate contact. That the transfer of motion from one body to another by impact is no less incomprehensible than actio in distans becomes apparent on a moment's reflection; and that the hypothesis of an intervening "ether"—itself composed of atoms, the interspaces between which are larger in proportion to these atoms than the interstellar spaces—is simply a new presentation of the old perplexity in a worse form, and in no wise helps to remove the difficulties involved in the phenomenon of the correspondence between the movements of two bodies without contact, is equally clear, and has been sufficiently pointed out by Herbert Spencer ("First Principles," p. 59). My object is merely to show that, if the validity of every theory of the constitution of matter is to be tested by our ability to realize it in thought—to bring it clearly before the scientific imagination, to represent it mentally as a distinct image, or whatever may be the form of words in which this requirement is expressed—the atomic theory fails as completely as any other theory of the nature of matter which has ever been propounded.
But what ground is there for the assumption that conceivability is