30. We start from the position that a broad line of demarcation must be drawn between the mental and the physical aspect of a process, supposing them to be identical in reality. Nothing can be more _unlike_ a logical proposition than the physical process which is its correlate; so that Philosophy has hitherto been forced to forego every attempt at an explanation of how the two can be causally connected: referring the connection to a mystery, or invoking two different agents, spiritual and material, moving on parallel lines, like two clocks regulated to work simultaneously. But having recognized this difference, can we not also discern fundamental resemblances? First and foremost, we note that there is common to both the basis in Feeling: they are both modes of Consciousness. The Mind thinking the logical proposition is not, indeed, in the same _state_ as the Mind picturing the physical process which is the correlate of that logical proposition--no more than I, who see you move on being struck, have the same feelings as you who are struck. But the Mind which pictures the logical proposition as a process, and pictures the physical process as a bodily change, is contemplating one and the same event under its subjective and objective aspects; just as when I picture to myself the feelings you experience on being struck I separate the subjective aspect of the blow from its objective aspect. Secondly, between the logical proposition and the physical process there is a community of causal dependence, i. e. the mode of grouping of the constituent elements, whereby this proposition, and not another, is the result of this grouping, and not another. In fact, what in subjective terms is called Logic, in objective terms is Grouping.
31. Let us approach the question on a more accessible side. Sensation avowedly lies at the basis of mental manifestations. Now, rightly or wrongly, Sensation is viewed alternately as a purely subjective fact--a psychological process--and as a purely objective fact--the physiological reaction of a sense-organ. It is so conspicuously a physiological process that many writers exclude it from the domain of Mind, assign it to the material organism, and believe that it is explicable on purely mechanical principles. This seems to me eminently disputable; but the point is noticed in proof of the well-marked objective character which the phenomenon assumes. In this aspect a sensation is simply the reaction of a bodily organ. The physiologist describes how a stimulus excites the organ, and declares its reaction to be the sensation. Thus viewed, and expressed in terms of Matter and Motion, there is absolutely nothing of that subjective quality which characterizes sensation. Yet without this quality the objective process cannot be a sensation. Exclude Feeling, and the excitation of the auditory organ will no more yield the sensation of Sound by its reaction, than the strings and sounding-board of a piano when the keys are struck will yield music to a deaf spectator. Hence the natural inference has been that inside the organism there is a _listener_: the Soul is said to listen, transforming excitation into sensation. This inference only needs a more systematic interpretation and it will represent the biological theory, which demands something _more_ than the reaction of the sensory organ--namely, the reaction of the whole organism _through_ the sensory organ. I mean, that no organ isolated from the organism is capable of a _physiological_ reaction--only of a physico-chemical reaction; and sensation depends on (_is_) the physiological reaction. When a sense-organ is stimulated, this stimulation is a vital process, and is raised out of the class of physico-chemical processes by virtue of its being the indissoluble part of a complex whole. Interfere with any one of the co-operant conditions--withdraw the circulation, check respiration, disturb secretion--and the sense-organ sinks from the physiological to the physical state; it may then be brought into contact with its normal stimuli, but no stimulation (in the vital sense) will take place, there will be no vital reaction.