[Bowman] FUNDAMENTAL PROCESSES IN HISTORICAL SCIENCE 141 
correct. By mixing them, however, in the manner described the first 
student, for instance, was bound to discover that a mixture of C alone 
with D alone would produce color, unless at this point in the test he 
took a quantity of B instead of a quantity of C or D; in which case, 
while under the impression that he was mixing C with D, he would in 
reality mix either B with C or B with D and he would still have a color- 
less mixture. Apart from such an error by the student or operator 
in applying his process of investigation, he would be led necessarily 
to the correct conclusion that a combination of C with D would produce 
color. 
Equally simple and more effective illustrations of the principle 
involved in the above definition of a correct process are supplied by the 
fundamental processes of mathematical science known as the four rules 
of arithmetic, e.g. the process of addition. In finding thesum of aseries 
of numbers the operator, if he applies the process of addition correctly, 
must necessarily get a correct result. By deviating from the require- 
ments of the process he may introduce an error into the result; but 
in the process itself, apart from such deviation, there is no room for 
error. 
iv. WHERE AN OPERATOR IS ACCREDITED AS APPLYING A CORRECT 
PROCESS, THOSE PERSONS WHO ARE NOT IN A POSITION TO 
JUDGE OF THE MATTER FOR THEMSELVES ARE REQUIRED AS 
REASONABLE MEN TO ACCEPT HIS RESULTS AS CORRECT UNLESS 
THE CONTRARY BE PROVEN.! 
Illustrations:—A correct process for solving the problem of erecting 
iron highway and railway bridges adequate for the strains which such 
bridges will be required to bear, is applied by expert engineers and their 
qualified assistants who are employed for the purpose by reliable 
bridge companies and are, therefore, accredited operators of this 
engineering process. Unless the contrary be proven, the results of 
their work are accepted as correct by all who have not the expert 
knowledge, or, having such knowledge, are nevertheless for other 
reasons not in a position, to judge of the matter for themselves. Thus a 
highway bridge so erected is used by the ordinary traveller in the 
confidence that it is safe unless an engineer as expert at least as the 


1 The principle is here defined in the form best adapted for illustration and ap- 
plication. Conciser and equally correct definitions of the principle are: (1) Where 
a correct process is applied by an accredited operator, his results must be accepted 
as correct unless the contrary be proven; and (2) A correct process and an accredited 
operator constitute prima facie evidence of a correct result. (Prima facie evidence: 
evidence that must prevail unless the contrary be proven.) 
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