66 
(1) Metals conductors. 
(2) Not-metals conductors. 
(3) Not-metals not-conductors. 
It comes to the same thing if we say that it excludes the 
existence of the class metals not-conductors. But every 
scientific process has its inverse process. As addition is 
undone by subtraction, multiplication by division, differen- 
tiation by integration, so logical induction is the inverse 
process of deduction. Given certain classes of objects, we 
endeavour by induction to pass back to the laws embodied 
in those classes. There does not exist indeed any distinct 
method of induction except such as consists in inverting the 
processes of deduction, by noting and remembering the laws 
from which certain effects necessarily follow. The difficul- 
ties of induction are thus exactly analagous to those of 
integration. 
As I have fully explained in my previous essays and 
papers, two terms or classes can be combined consistently 
with the laws of thought in four different ways. Now out of 
four such combinations sixteen selections (two to the power 
four) can be made. As each distinct law gives a different 
series of combinations, it follows that there could not pos- 
sibly exist more than sixteen distinct forms of law governing 
the combinations of two classes. But in one case, where all 
the combinations remain, no special law applies; in other 
cases it can be shown that the combinations remaining are 
so few as to imply self-contradiction. Only six sets of com- 
binations require further consideration. By deductive exa- 
mination it is found that four of these cases correspond to 
varieties of the general form of law, A = AB, which ex- 
presses the inclusion of the class A in the class B. By the 
introduction of negative terms this general form may 
receive four essentially different logical variations. Thus 
we have 
