"What we directly apprehend," writes Professor Helmholtz, "is not the immediate action of the external exciting cause upon the ends of our nerves, but only the changed condition of the nervous fibres which we call the state of _excitation_ or functional activity." And further on:--"The simple rule for all illusions of sight is this: _we always believe that we see such objects as would, under conditions of normal vision, produce the retinal image of which we are actually conscious_. If these images are such as could not be produced by any normal kind of observation, we judge of them according to their nearest resemblance; and in forming this judgment, we more easily neglect the parts of sensation which are imperfectly than those which are perfectly apprehended. When more than one interpretation is possible, we usually waver involuntarily between them; but it is possible to end this uncertainty by bringing the idea of any of the possible interpretations we choose as vividly as possible before the mind by a conscious effort of the will." Helmholtz on _The Recent Progress of the Theory of Vision_. pp. 230, 31 and p. 307. [104] Two acute reasoners, who will be alternately acquitted of madness by contending schools of thought, have arrived at conclusions very favourable to the sanity of idealizing men. In his first lecture at the Royal Institution, Professor Masson spoke in the following terms of _Hume_ and _Fichte_. "There is the system of Nihilism, or, as it may be better called, Non-Substantialism. According to this system, the Phænomenal Cosmos, whether regarded as consisting of two parallel successions of phænomena (Mind _and_ Matter), or of only one (Mind _or_ Matter), resolves itself, on analysis, into an absolute Nothingness,--mere appearances with no credible substratum of Reality; a play of phantasms in a void. If there have been no positive or dogmatic Nihilists, yet both Hume for one purpose, and Fichte for another, have propounded Nihilism as the ultimate issue of all reasoning that does not start with some _à priori_ postulate."--Recent British Philosophy, p. 66. The reader will observe that to raise the question fully, we have spoken of the special form of Idealism to which Mr. Mill gives the first place in his description, (Examination of Hamilton's Philosophy, p. 8.) "According to one of the forms, the sensations which, in common parlance, we are said to receive from objects, are not only all that we can possibly know of the objects, but are all that we have any ground for believing to exist.----Those who hold this opinion are said to doubt or deny the existence of matter. They are sometimes called by the name Idealists, sometimes by that of Sceptics, according to the other opinions which they hold. They include the followers of Berkeley and those of Hume. Among recent thinkers, the acute and accomplished Professor Ferrier, though by a circuitous path, and expressing himself in a very different phraseology, seems to