The two statements that best explain the reasoning behind his ultimate decision are:
. . . I felt that there was some justice in his argument as his tale and the feelings he now expressed proved him to be a creature of fine sensations and did I not as his maker owe him all the portion of happiness that it was in my power to bestow?
After a long pause of reflection, I concluded that the justice due both to him and my fellow creatures demanded of me that I should comply with his request.
This refers to the type of feeling where a person is indecisive about what he feels as he has conflicting emotions about something.
Hence, we can see that based on the mixed feelings which Dr. Frankenstein wrestles with, his reasoning behind his ultimate decision is stated above.
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