IRRITABILITY AS A SIGN OF LIFE 3 
we possess. Thus a part of the study which follows has 
been made upon such simple things as seeds and garden 
peas. It is surprising how closely the results obtained 
with these parallel those obtained from nerves. 
The really important and peculiar property of living 
things is the psychic life they show. And if we actually 
had an accurate means of testing the degree or amount 
of life, it would be some kind of a reagent or instrument 
for testing “‘psychism,” as we may call it. But un- 
fortunately we cannot at present find any means of test- 
ing this property. We do not know what its physical 
basis is, and, until we discover that, we cannot make 
a psychometer which we can apply to all kinds of 
living and non-living things, and thus measure the 
arnount of psychism, and hence of life, which they 
possess. In the absence of any such psychometer we 
have to do the best we can, and take as a measure of 
this property those physical and chemical changes which 
experience or experiment demonstrates to us always 
accompany the psychic change. The situation is very 
much as it was in the realm of electricity before the 
galvanometer was invented; an idea of the quantity 
of electricity produced by a battery could be obtained 
only indirectly by measuring the amount of chemical 
change which the current produced, since Faraday 
found that that amount was always a measure of the 
amount of electricity. 
There are material changes which occur in living 
things as long as they are alive and show psychic life 
of any kind. The changes which we may rely upon to 
measure the amount of life, and thus indirectly the 
amount of psychism, are partly visible changes, but in 
