t and fresh ones afflict mankind: all these
things doubtless have their causes, but if we do not know what they are,
we have no measure of the effects, and cannot tell when or where they
will happen.

Laws of Concomitant Variations may hold good only within certain limits.
That bodies contract as the temperature falls, is not true of water
below 39 deg. F. In Psychology, Weber's Law is only true within the median
range of sensation-intensities, not for very faint, nor for very strong,
stimuli. In such cases the failure of the laws may depend upon something
imperfectly understood in the collocation: as to water, on its molecular
constitution; as to sensation, upon the structure of the nervous system.

Sec. 4. Secondary Laws, again, are either of Succession or of Co-existence.

Those of Succession are either--(1) of direct causation, as that 'Water
quenches fire,' or (more strictly) that 'Evaporation reduces
temperature'; or (2) of the effect of a remote cause, as 'Bad harvests
tend to raise the price of bread'; or (3) of the joint effects of the
same cause, as that 'Night follows day' (from the revolution of the
earth), or the course of the seasons (from the inclination of the
earth's axis).

Laws of Co-existence are of several classes. (1) One has the generality
of a primary law, though it is proved only by Agreement, namely, 'All
gravitating bodies are inert'. Others, though less general than this,
are of very extensive range, as that 'All gases that are not decomposed
by rise of temperature have the same rate of expansion'; and, in Botany
that 'All monocotyledonous plants are endogenous'. These laws of
Co-existence are concerned with fundamental properties of bodies.

(2) Next come laws of the Co-existence of those properties which are
comprised in the definitions of Natural Kinds. Mill distinguished
between ([alpha]) classes of things that agree among themselves and differ
from others only in one or a few attributes (such as 'red things,'
'musical notes', 'carnivorous animals', 'soldiers'), and ([beta]) classes of
things that agree among themselves and differ from others in a multitude
of characters: and the latter he calls Natural Kinds. These comprise the
chemical elements and their pure compounds (such as water, alcohol,
rock-salt), and the species of plants and animals. Clearly, each of
these is constituted by the co-existence or co-inherence of a multitude
of properties, some of which are selected as the bas