al, but merely
as a special mode of the uniformity of Nature or experience.

(6) Certain Uniformities of Co-existence; but for want of a general
principle of Co-existence, corresponding to Causation (the principle of
Succession), we can only classify these uniformities as follows:

(a) The Geometrical; as that, in a four-sided figure, if the opposite
angles are equal, the opposite sides are equal and parallel.--Countless
similar uniformities of co-existence are disclosed by Geometry. The
co-existent facts do not cause one another, nor are they jointly caused
by something else; they are mutually involved: such is the nature of
space.

(b) Universal co-inherences among the properties of concrete
things.--The chief example is the co-inherence of gravity with inertia
in all material bodies. There is, I believe, no other entirely
satisfactory case; but some good approximations to such uniformity are
known to physical science.

(c) Co-existence due to Causation; such as the positions of objects in
space at any time.--The houses of a town are where they are, because
they were put there; and they remain in their place as long as no other
causes arise strong enough to remove or destroy them. Similarly, the
relative positions of rocks in geological strata, and of trees in a
forest, are due to causes.

(d) The co-inherence of properties in Natural Kinds; which we call the
constitution, defining characters, or specific nature of such
things.--Oxygen, platinum, sulphur and the other elements; water, common
salt, alcohol and other compounds; the various species of plants and
animals: all these are known to us as different groups of co-inherent
properties. It may be conjectured that these groupings of properties are
also due to causation, and sometimes the causes can be traced: but very
often the causes are still unknown; and, until resolved into their
causes, they must be taken as necessary data in the investigation of
nature. Laws of the co-inherence of the properties of Kinds do not, like
laws of causation, admit of methodical proof upon their own principles,
but only by constancy in experience and statistical probability (c. xix,
Sec. 4).

(e) There are also a few cases in which properties co-exist in an
unaccountable way, without being co-extensive with any one species,
genus, or order: as most metals are whitish, and scarlet flowers are
wanting in fragrance. (On this Sec. 7, see Venn's _Empirical Logic_, c. 4.)

Sec. 8.