(B) _First trial._ _Problem_: Unprepared home work. _Antecedents_ _Consequents_ 1. Length of lesson, } 2. Definiteness of lesson, } Work _not_ properly 3. Amount of interest shown, } prepared. 4. Physical condition the same. } _Second trial._ 1. _Length_ of lesson the same, } 2. _Lesson made more definite_, } Work _properly_ prepared. 3. _Interest_ the same, } 4. Physical _condition_ the same. } The foregoing symbolized: L D I C W L ‒ I C ‒ It is seen that indefiniteness of lesson assignment is the cause of the unprepared home work. (4) Advantages and disadvantages of the Method of Difference. The main difficulty attending the use of the method of difference is the _complexity_ of phenomenon. The very nature of the method insists as an essential requirement that _only one material antecedent shall be varied at a time_. In life the variations are more or less confused, and it is often not only impossible to observe cases of a single variation, but frequently error comes through _overlooking_ antecedents which are material to the case under investigation. For these reasons the Method of Difference is more a method of _experiment_ than it is a method of observation. By controlling the circumstances it becomes possible to vary but _one_ antecedent at a time, and also to bring into prominence _all_ of the _material_ antecedents. Bacon claims that _all “crucial instances”_ are merely applications of the Method of Difference. By crucial instance he means any fact which will enable us to determine _at once_ which supposition is the correct one. For example, the physician may not know whether it is malaria or typhoid fever till he takes a blood test; such a test typifies “crucial instances.” The various tests in chemistry are likewise cases of crucial instances, and, in consequence, this science makes use of “Difference” more than any other method. (5) Characteristic features of Method of Difference. There are three distinguishing marks of the Method of Difference: these are, (1) The phenomenon does not always happen; (2) _One_ antecedent is variable; (3) The other antecedents are more or less invariable. The following symbolizations will make these three characteristics evident: _Antecedents_ _Consequents_ (1) A B C P A ‒ C ‒ (2) ‒ B C ‒ X B C P (3) L M T K P L M ‒ K ‒ AGREEMENT AND DIFFERENCE COMPARED.