spark is enough to reduce the whole mass of wood to ashes. But in the case of bricks
and mortar there is no potentiality to react in this manner and, therefore, when the
match is applied nothing happens. Change, in such a case, therefore, takes place ac-
cording to the potentialities existing in the material and follows the tendency of natural
forces under the given conditions. If the conditions change, the tendencies may also
change and an entirely new kind of change may be brought about. For example, the
wood may be subjected to the action of certain chemical substances and converted into
charcoal. Let us take another example. A breeder can evolve a new species of dog by
the appropriate crossing of different kinds of dogs but he cannot produce a new species
of cat in this manner. The potentiality for the production of a new species of cat does
not exist in the latter case. Here again, therefore, we are bound by limitations set by
natural tendencies and potentialities and cannot go against them. It is true that if we
have the necessary knowledge we can, by the introduction of new factors, bring about
changes which appeared impossible before, but that does not mean that we have vio-
lated the fundamental law of Nature referred to above.
- The incidental cause does not move or stir up the natural tendencies into
activity; it merely removes the obstacles, like a farmer (irrigating a field).
The idea embodied in IV-2 is further elaborated in IV-3. Transformation from
one kind into another takes place, as we have seen, according to the resultant effect of
all forces involved. Everything has the potentiality of changing in a number of direc-
tions and by bringing to bear upon it different kinds of forces, we can make it change
in one or another of the directions as illustrated in the following diagram.