Gustav Fechner, a German, was both a scientist and a mystic.
"The many hours spent in meditation and reflection during these years deepened Fechner's religious awareness and his concern over the problem of the soul. He turned to philosophy and began to direct the full force of his genius toward the question of the relationship between the mind and the body. He decided that the two are identical; both are aspects of the same fundamental unity...
On the morning of October 22, 1850--an important date in the history of psychology--Fechner had an insight that the law of the connection between the mind and the body can be found in a statement of quantitative relation between mental sensation and material stimulus."
From this inspiration Fechner invented 'psychophysics', a very
useful study of the relationship between stimulus intensity and
subjective experience (detection) of the stimulus. The techniques
he invented opened the door to the scientific study of mind, combining
manipulation of quantifiable, physical, stimuli with subjective
reports of stimulus perception. The success of this approach influenced
Wilhelm Wundt, who modified the techniques to create the first
scientific study of conscious experience.