Occult Science in India, by Louis Jacoilliot, , at sacred-texts.com
In order to make ourselves understood, where there is as yet no accepted mode of speech, we will say what we mean by the term "spirit force."
By "spirit force" we mean the alliance between the intellect and the physical forces, in order to act upon inanimate objects, without pre-determining, in any way, the cause which sets this force in motion.
The meaning of the word is not strictly, perhaps, that which is generally attached to it. We will therefore say that we use it only to classify the phenomena which we are about to describe, and that the meaning here given expresses accurately the signification of the term used by the Hindus.
The supreme cause of all phenomena, according to the Brahmins, is the pure agasa fluid, or the vital fluid, which is diffused throughout nature, and puts animate or inanimate, visible or invisible beings, in communication with each other. Heat, electricity, all the forces of nature, in short, are but modes of action and particular states of this fluid.
The being who possesses an excess of this vital fluid acquires a proportionate power, both over animate beings not so highly favored, and over inanimate beings. The spirits themselves are sensible to the influence of this universal fluid, and can place their power at the service of those who are able to evoke them.
According to some Brahmins, agasa is the moving thought of the universal soul, directing all souls, who would
be in constant communication with each other, if the gross envelope of the body did not in a measure prevent. Thus, the more completely the soul disentangles itself from its vestmentthe bodyby contemplation, the more sensible it becomes to this universal fluid, whereby all beings, whether visible or invisible, are united.
Such is the theory. We merely set it forth and propose to confine ourselves to the rôle of an interpreter and nothing more.