Its general tenor is as
follows: The physical cannot be referred back to the psychical, nor the
psychical to the physical. Both orders of phenomena run side by side as
parallels that never separate. Both represent a concatenation of causes
complete in itself, that is never broken, or interrupted, or completed.
And in both there is real causality. Thought really causes thoughts and
feelings. Movement really causes movements. But the one series is always
strictly correlated with the other, and corresponds with it. And thus all
existence is double, and man is an obvious illustration of this. To every
thought, feeling, or exercise of will there corresponds some excitement,
movement or change in the body. I will: my arm moves. Subtle nervous
processes run their course in my brain, and I think. That I will has its
sufficient reasons, its causes lie entirely in the preceding state of my
mind, in motives of feeling, in ideas which again have their efficient
causes in a previous psychical condition, and so on. And that my arm moves
has its efficient cause in the stored-up energies of the muscle-substance,
in the stimulus and impulse conveyed by the motor nerve from the brain.
And these conditions have their purely physiological causes and reasons
again in preceding purely physiological states and processes. (It goes
without saying that a mechanical theory of life is the necessary
presupposition of this parallelistic theory.) But both sets of processes
correspond exactly one to another, and the first is only the inner aspect
of the second, and the second the outer aspect of the first. Thus it is
quite true that my arm moves when I will. But in reality it is quite as
true to say that when my arm moves I will. But we must not substitute
"because" for "when." This theory must maintain, and does maintain, that
even the most abstract and subtle ideas, the deepest processes of
consciousness, have some corresponding bodily processes, either in the
brain or in the nervous substance generally, and, on the other hand, that
no physical process is without this psychical inwardness. The result is
that this inwardness and soul are attributed also to the purely material
world, the world of "dead" matter. In this way it is believed that
everything gets its due; the thorough mechanical explicability of bodily
phenomena, and the law of the conservation of energy and of matter, and,
on the other hand, very decisively also, the independence and unique