same. To-day the same idea is better expressed in the
doctrine of the eternity of mass and the conservation of energy. In
plain language, this doctrine teaches that every change in every part of
the physical world, every motion in matter, must be preceded by physical
conditions which may be regarded as the equivalent of the change in
question.

But this makes the physical world a closed system, a something complete
in itself. Where is there room in such a system for minds?

It does indeed seem hard to find in such a system a place for minds, if
one conceives of minds as does the interactionist. We have seen (section
36) that the interactionist makes the mind act upon matter very much as
one particle of matter is supposed to act upon another. Between the
physical and the mental he assumes that there are _causal_ relations;
_i.e._ physical changes must be referred to mental causes sometimes, and
mental changes to physical. This means that he finds a place for mental
facts by inserting them as links in the one chain of causes and effects
with physical facts. If he is not allowed to break the chain and insert
them, he does not know what to do with them.

The parallelist has not the same difficulty to face. He who holds that
mental phenomena must not be built into the one series of causes and
effects with physical phenomena may freely admit that physical phenomena
form a closed series, an orderly system of their own, and he may yet find
a place in the world for minds. He refuses to regard them as a part of
the world-mechanism, but he _relates_ them to physical things, conceiving
them as _parallel to_ the physical in the sense described (sections
37-39). He insists that, even if we hold that there are gaps in the
physical order of causes and effects, we cannot conceive these gaps to be
filled by mental phenomena, simply because they are mental phenomena.
They belong to an order of their own. Hence, the assumption that the
physical series is unbroken does not seem to him to crowd mental
phenomena out of their place in the world at all. They must, in any
case, occupy the place that is appropriate to them (section 38).

It will be noticed that this doctrine that the chain of physical causes
and effects is nowhere broken, and that mental phenomena are related to
it as the parallelist conceives them to be, makes the world-system a very
orderly one. Every phenomenon has its place in it, and can be accounted
for, wh