In philosophy, is a subtile fluid, supposed by sir Issac Newton to fill all space, and to be the cause of gravitation and numerous other phenomena, inexplicable on other grounds. Sir Isaac, however, only suggested it as a probable cause of these phenomena, or in other words observed, that, if there were such a fluid with given properties, it may be the cause of the appearances mentioned. Such, however, was the character of this eminent philosopher, that his suppositions have been ranked with the demonstrations of other authors.

We shall only observe at present, that those who examine most minutely the operations of nature will be the least ready to reject this idea. A medium very different from the grosser ones which are subject to our senses appears necessary to explain the phenomena of sound, light, gravitation, electricity, Galvanism, etc. Let it have the properties assigned to it, or any other, some very subtile medium probably exists. That this aether is the nervous fluid, as some authors have contended, can neither be admitted nor denied. The whole, at present, is gratuitous. No facts have even proved its existence: it is the tortoise which, in our present views, must be put under the elephant; but we seem fast approaching to some further knowledge of these invisible fluids, and in the progress of this work may make some further steps in the enquiry.

See Nerves, and Nervous fluid.