2 THE NATURAL HISTORY OF THE COMMON CRAYFISH. 
for the commonest purposes of everyday existence. 
Common sense is science exactly in so far as it fulfils 
the ideal of common sense; that is, sees facts as they 
are, or, at any rate, without the distortion of prejudice, 
and reasons from them in accordance with the dictates 
of sound judgment. And science is simply common sense 
at its best; that is, rigidly accurate in observation, and 
merciless to fallacy in logic. 
Whoso will question the validity of the conclusions of 
sound science, must be prepared to carry his scepticism 
a long way; for it may be safely affirmed, that there is 
hardly any of those decisions of common sense on 
which men stake their all in practical life, which can 
justify itself so thoroughly on common sense principles, 
as the broad truths of science can be justified. 
The conclusion drawn from due consideration of the 
nature of the case is verified by historical inquiry; and 
the historian of every science traces back its roots to the 
primary stock of common information possessed by all 
mankind. 
In its earliest development knowledge is self-sown. 
Impressions force themselves upon men’s senses whether 
they will or not, and often against their will. The 
amount of interest which these impressions awaken is 
determined by the coarser pains and pleasures which 
they carry in their train, or by mere curiosity; and 
reason deals with the materials supplied to it as far as 
that interest carries it, and no farther, Such common 
