The transmission hypothesis advanced by James is a modification of the soul hypothesis. The transmission hypothesis postulates the existence of a physical world and of an independent universe of consciousness. Consciousness, however, cannot manifest itself in this sublunar world without the occurrence of definite physical changes. That level of physical changes which makes the manifestations of consciousness possible is termed the physical threshold. Now the ocean of consciousness pours forth its psychic waves into the material world with the rise and fall of the physical threshold. The threshold is to be pictured as a sort of flood gate regulating the volume and intensity of the transmitted current. The rising of the threshold diminishes the psychic stream, while the lowering of the threshold permits a greater volume of consciousness to pour over into our physical world.
The transmission hypothesis has certain advantages over the previous ones discussed by us. While this hypothesis postulates the independence of consciousness, it is also in accord with the scientific proposition now generally accepted, namely that mental life is somehow connected with or is a function of brain activity, only specifying that this function is one of transmission. It claims to fall in line with the threshold concept of psychophysics as worked out by Fechner, and further harrowed by the "new psychology" movement; moreover, it is comprehensive enough to embrace all the facts and speculations brought out by recent investigations in the domain of mental pathology.
The transmission hypothesis, however, has also disadvantages which are of such a grave nature as to make one hesitate to accept it. The transmission hypothesis from its very nature is unverifiable. For, if, by hypothesis, consciousness manifests itself in this sublunar world (the only one we know) only under physical conditions, how can we ever come to know and verify a postulated world of pure consciousness? Being outside the domain of our psychophysical world, the universe of disembodied consciousness cannot, by hypothesis, furnish us the means for its verification. In this sublunar world we can know of the existence of consciousness through its physical expressions, through its being embodied. How then, can we ever reach a universe of disembodied consciousness? But a hypothesis which from its very nature is not verifiable cannot possibly be accepted.
The transmission hypothesis is all the more unacceptable as the terms in which it is expressed are contradictory, and the analogy on which it is based is essentially illegitimate. Consciousness is supposed to be different in nature from the physical world and existing independently, the psychophysical threshold alone regulating the volume of the stream of consciousness to be poured over into the material world. The threshold then which is physical in character limits consciousness, but how can the two be limited by each other when they are totally different in nature? In assuming two different universes, we assert that the two cannot limit each other, but in examining again the concept of threshold we make a contradictory assertion that the two can and do limit each other.
The very analogy on which the concept of "transmission function" is based is illegitimate when applied to consciousness in its relation to the physical world. The concept of "transmission function" can only be applied to a case where the transmitter and the thing transmitted are of homogeneous terms, but not where the terms are essentially heterogeneous. A stream of liquid can be transmitted through a pipe, a beam of light through stained glass, or a Roentgen ray through soft or more or less rarified cellular tissue. Both the transmitter and the material transmitted are physical in their nature, but how can an idea or feeling such as our idea of eternity, or of infinity, or aesthetic, or moral sense be transmitted through a tube? How then can we apply the concept of transmission-function to consciousness and the physical world where the two are totally different in nature? The analogy is figurative and scientifically illegitimate.