502 
PEOFESSOE JEVONS ON THE MECHANICAL 
But the first premise informs us that iron is a metal, and thus excludes the combinations 
(y) and (&), while the second premise informs us that metal must be element , and thus 
further excludes the combination ((3). It follows that iron must be described by the 
first alternative (a) only, and that it is an element, thus [proving the conclusion of the 
syllogistic mood Barbara. 
16. In employing this method of inference, it is soon found to be tedious to write out 
at full length in words the combinations of terms to be considered. It is much better to 
substitute for the words single letters, A, B, C, &c., which may stand in their place and 
bear in each problem a different meaning, just as x, y, z in algebra signify different 
quantities in different problems, and are really used as brief marks to be substituted for 
the full descriptions of those quantities. At the same time it is convenient to substitute 
for the corresponding negative terms small italic letters, a, b, c, &c. ; thus 
if A denote iron, a denotes what is not-iron. 
B „ metal , b „ „ not-metal. 
C „ element , c „ „ not-element. 
When these general terms are combined side by side, as in A B C, a B C, they denote 
a term or thing combining the properties of the separate terms. Thus ABC denotes 
iron which is metal and element ; a B C denotes metal which is element but not iron. 
These letter terms A, B, C, a, b, c, Sec. can, in short, be joined together in the manner of 
adjectives and nouns. 
17. I must particularly insist upon the fact, however, that there is nothing peculiar 
or mysterious in these letter symbols. They have no force or meaning but such as they 
derive from the nouns and adjectives for which they stand as mere abbreviations, 
intended to save the labour of writing, and the want of clearness and conciseness attach- 
ing to a long clause or series of words. In the system put forth by Boole various 
symbols of obscure or even incomprehensible meaning were introduced ; and it was im- 
plied that the inference came from operations different from those of common thought 
and common language. I am particularly anxious to prevent the misapprehension that 
the method of inference embodied in the machine is at all symbolic and dark, or differs 
from what the unaided human mind can perform in simple cases. 
18. Great clearness and brevity are, however, gained by the use of letter terms; for 
if we take 
A = iron, 
B = metal, 
C = element; 
then the premises of the problem considered are simply 
Iron is metal . . . A is B, 
Metal is element . . B is C. 
• ( 1 ) 
• ( 2 ) 
