Edward Bradford Titchener, answered from the texts and cited to the page.
When I explain a conscious process by reference to the nervous system, I do not mean that the brain causes the mind or the mind the brain.1 Mind and body are two aspects, or two parallel series, that run in exact correspondence but never cross: for every conscious process there is a concurrent nervous process, and the two vary together, point for point, but neither acts upon the other.
This is psychophysical parallelism, and I hold it because the alternatives fail. Interaction — the mind pushing the body, the body pushing the mind — violates the closed causal order of the physical world and explains nothing. What, then, does the nervous system give the psychologist? Not a cause of consciousness, but its systematic ground: the parallel physiological process supplies the connected, law-governed background against which the disconnected data of introspection may be arranged and explained.
I invoke the nervous process not to reduce mind to matter but to give my descriptions a systematic order they could not have from introspection alone.2345
Nevertheless, it is by reference to the body, to the nervous system and the organs attached to it, that we explain mental phenomena. The nervous system does not cause,- but it does explain mind. It explains mind as the map of ^ country explains the fragmentary glimpses of hills and rivers and towns that we catch on our journey through it.A Text-book of Psychology, p. 39
He thus learns to formulate the laws of connection of the elementary mental processes. If sensations of tone occur together, they blend or fuse; if sensations of colour occur side by side, they enhance one another: and all this takes place in a perfectly regular way, so that we can write out laws of tonal fusion and laws of colour contrast.A Text-book of Psychology, pp. 38–39
It is true that organic sensations, often pleasant or unpleasant, are constituents of all my consciousnesses; and that I am always called by the same name, always treated as the same self.An Outline of Psychology, p. 342
And the psychologist, in the same way, describes and measures—so far as he is able to measure—the phenomena of his world, without assuming any active or perduring mind in the background; for him, mind is simply the =inclusive name of all these phenomena=. That is the first rough answer to our question.A Beginners Psychology
It has been seriously argued by some psychologists that mind appears wherever life appears; not only in the animal kingdom, but in the vegetable as well. This is a question which we can- not stop to discuss here. At any rate the plant-mind, if there is such a thing, must be so extraordinarily rudimentary and so totally different from our own that it is hopeless to try to form any idea of it. 7.A Primer of Psychology
Psychophysical parallelism was the standard mind-body position of late-nineteenth-century experimental psychology and Titchener held it in a considered form, using the nervous process as the systematic ground of psychological explanation without reducing mind to it. The position has since been largely abandoned in the philosophy of mind in favor of identity theories, functionalism, and various forms of interaction and supervenience, and parallelism's clean two-series picture is generally rejected as both metaphysically extravagant and empirically idle. Titchener's use of parallelism was more methodological than metaphysical — a way of ordering psychological data — and in that limited role it prefigures the modern reliance on neural correlates of consciousness, even as his strict two-parallel- series metaphysics is not held today.