Now, according to the psycho-physiological theory of memory, with every passing state of conscious experience, with every idea, thought, or perception, the brain process that goes along with it leaves some trace, some residue of itself, within the neurons and in the functional arrangements between them. It is an accepted principle of physiology that when a number of neurons, involved, let us say, in a coördinated sensori-motor act, are stimulated into functional activity they become so associated and the paths between them become so opened or, as it were, sensitized, that a _disposition_ becomes established for the whole group, or a number of different groups, to function together and reproduce the original reaction when either one or the other is afterward stimulated into activity. This “disposition” is spoken of in physiological language as a lowering of the threshold of excitability—a term which does not explain but only describes the fact. For an explanation we must look to the nature of the physical change that is wrought in the neurons by the initial functioning. This change we may speak of as a _residuum_. Similarly a system of brain neurons, which in any experience is correlated in activity with conscious experience, becomes, so to speak, sensitized and acquires, in consequence, a “disposition” to function again as a system (lowering of thresholds?) in a like fashion; so that when one element in the system is again stimulated it reproduces the whole original brain process, and with this reproduction (according to the doctrine of psycho-physical parallelism) there is a reproduction of the original conscious experience. In other words, without binding ourselves down to absolute precision of language, it is sufficiently accurate to say that every mental experience leaves behind a residue, or a trace, of the physical brain process in the chain of brain neurons. This residue is the physical _register_ of the mental experience. _This physical register may be conserved or not._ If it is conserved we have the requisite condition for memory; the _form_ in which our mental experiences are conserved. But it is not until these physical registers are stimulated and the original brain experience is reproduced that we have memory. If this occurs the reproduction of the brain experience reproduces the conscious experience, i.e., conscious memory (according to whatever theory of parallelism is maintained). Thus in all ideation, in every process of thought, the record of the conscious stream may be registered and conserved in the correlated neural process. Consequently, the neurons in retaining residua of the original process become, to a greater or less degree, _organized into a functioning system_ corresponding to the system of ideas of the original mental process and capable of reproducing it. When we reproduce the original ideas in the form of memories it is because there is a reproduction of the physiological neural process.