In the same lecture Tait gave a succinct statement of his philosophy of knowledge. "In dealing with physical science" he said, "it is absolutely necessary to keep well in view the allimportant principle that Nothing can be learned as to the physical world save by observation and experiment, or by mathematical deductions from data so obtained. On such a text volumes might be written, but they are unnecessary, for the student of physical science feels at each successive stage of his progress more and more profound conviction of its truth. He must receive it, at starting, as the unanimous conclusion of all who have in a legitimate manner made true physical science the subject of their study, and, as he gradually gains knowledge by this—the only—method, he will see more and more clearly the absolute impotence of all so-called metaphysics or a priori reasoning, to help him to a single step in advance. Man has been left entirely to himself as regards the acquirement of physical knowledge. But he has been gifted with various senses (without which he could not even know that the physical world exists) and with reason to enable him to control and understand their indications. Reason, unaided by the senses, is totally helpless in such matters. The indications given by the senses, unless interpreted by reason, are utterly unmeaning. But when reason and the senses work harmoniously together, they open to us an absolutely illimitable prospect of mysteries to be explored."
What, it may be asked, is this reason which interprets the indications of the senses? Is it not the very a priori knowledge which the rational philosophers have ascribed to the mind? If so, why all this tirade against so-called metaphysics and a priori reasoning? To one who held that all knowledge came through the senses, such procedure would be logical, but not to the savant who uttered the above theory of knowledge. The speculations in the Unseen Universe assume the truth of the vortex theory of atoms. According to the ancient idea of the atom, it is a hard incompressible sphere. Boscovich removed the idea of hardness, and reduced the atom to a mere centre of force. Rankine, we have seen, supposed the point surrounded
