ll
greater proportion will be irritable of those who are both invalids and
poets.

On the whole, from the discussion of probabilities there emerge four
principal cautions as to their use: Not to make a pedantic parade of
numerical probability, where the numbers have not been ascertained; Not
to trust to our feeling of what is likely, if statistics can be
obtained; Not to apply an average probability to special classes or
individuals without inquiring whether they correspond to the average
type; and Not to trust to the empirical probability of events, if their
causes can be discovered and made the basis of reasoning which the
empirical probability may be used to verify.

The reader who wishes to pursue this subject further should read a work
to which the foregoing chapter is greatly indebted, Dr. Venn's _Logic of
Chance_.

CHAPTER XXI

DIVISION AND CLASSIFICATION

Sec. 1. Classification, in its widest sense, is a mental grouping of facts
or phenomena according to their resemblances and differences, so as best
to serve some purpose. A "mental grouping": for although in museums we
often see the things themselves arranged in classes, yet such an
arrangement only contains specimens representing a classification. The
classification itself may extend to innumerable objects most of which
have never been seen at all. Extinct animals, for example, are
classified from what we know of their fossils; and some of the fossils
may be seen arranged in a museum; but the animals themselves have
disappeared for many ages.

Again, things are classed according to their resemblances and
differences: that is to say, those that most closely resemble one
another are classed together on that ground; and those that differ from
one another in important ways, are distributed into other classes. The
more the things differ, the wider apart are their classes both in
thought and in the arrangements of a museum. If their differences are
very great, as with animals, vegetables and minerals, the classing of
them falls to different departments of thought or science, and is often
represented in different museums, zoological, botanical, mineralogical.

We must not, however, suppose that there is only one way of classifying
things. The same objects may be classed in various ways according to
the purpose in view. For gardening, we are usually content to classify
plants into trees, shrubs, flowers, grasses and weeds; the ordinary
crops of Engli