3 THE SOCIAL THEORY OF GEORG SIMMEL 
and the whole structure is possible at all, cannot be proved 
geometrically. The whole of geometry is therefore not 
valid in the same sense as its single propositions are valid. 
While the latter can be proved within the system one by 
another, the whole is valid only in relation to something 
external, in relation to the nature of space and our cate- 
gories of thought and perception. 
This mutual determination which gives the elements 
within a body of theoretic knowledge the significance of 
truth seems as a totality to be born of a new relativity, a 
relativity between the practical and the theoretical inter- 
ests of our lives. Our ideas of existential actuality are 
functions of a specific psycho-physiological character 
and are by no means merely mechanical reproductions. 
As different biological species equipped with different 
sense organs must have different ideas of the universe, it 
follows that none of these can be a pure reproduction of 
external nature. Each of these species, however, is able 
to survive and to adapt itself to its environment. The 
truth, therefore, can in essence mean nothing else but that 
idea or representation which guides a particular organism 
in the application of its forces toward useful results. An 
idea is not useful because it is true, but it is true because it 
is useful. It is true because it leads to a useful relation to- 
ward actuality, because it permits adaptation. 
There are therefore in principle just as many kinds of 
truth as there are different organisms and life-conditions. 
What is true for the eagle may not be true for the insect 
with facet-shaped eyes, and vice versa. 
These kinds of truth, however, do not lack a norma- 
tive fixity for the specific species. In so far as the organism 
and its constitution and needs are given on the one hand, 
and objective actuality on the other hand, the truth for 
that particular organism is ideally determined. There are