INTRODUCTORY 
25 
shows that the results reached on the basis 
of a discriminating mental abstraction (that 
is the imagined separation of things that 
may be inseparable) hold with approximate 
accuracy. A brain specialist may study the 
brain alone, broadly speaking, though no brain 
could survive when severed from the body with 
which it was united. Similarly we may study 
the economic activities of a group of people, 
ignoring their religious and political instincts 
as such, that is, so far as they are not expressed 
economically, though these instincts are an 
integral part of human nature ; or, again, we 
may study the economic activities of one 
person, mentally separating him from the 
community of which he forms a part. 
We may go even further in our defence 
of analytical economics, for there is this very 
important point to bear in mind, that many 
of the mental abstractions that we make 
as economists do not assume, even for the 
sake of argument, the isolated existence 
of things that cannot exist in isolation. 
Marginal abstractions merely confine us to 
noticing how changes in relation to a thing 
affect that thing. In other words, thinking 
now of the marginal method in economics 
merely, our abstractions consist merely in 
focusing attention on a thing and difieren-