CHAPTER VI 
CONCLUSIONS 
Summary.—While we cannot define life in a physical 
sense, for the reason that we have no measure of the 
psychic phenomena shown by living things, and these 
psychic phenomena are, after all, the most important 
of the characteristics of life, there are nevertheless certain 
phenomena associated always with the living processes 
which are so characteristic that for the majority of 
organisms with which we are familiar we have no diffi- 
culty in determining whether they are living or dead. 
Irritability is the universal sign of life, and by it living 
matter adjusts itself to its environment. The sign 
of this irritability is the functional power of the tissues. 
Thus by measuring the functional power we can speak 
of measuring the amount of irritability. The changes 
of a physical or chemical kind which accompany this 
functioning are very important for an understanding of 
the living process, for when we know them completely we 
shall probably understand the nature of irritability itself. 
In chapter ii we showed how it happened that, because 
of the apparent exception in the case of nerves, it has 
been generally concluded that chemical changes could 
not be considered to be essential to all living processes. 
Some of these changes, it appeared, must be due solely 
to physical processes, and for this reason irritability 
had come to be regarded as a purely physical phenome- 
non. Various hypotheses had been made to explain how 
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