Henry Sidgwick (1838-1900) on Reasoning to First Principles

I may begin by regarding some limited and qualified statement as self-evident, without seeing the truth of the simpler and wider proposition of which the former affirms a part; and yet, when I have been led to accept the latter, I may reasonably regard this as the real first principle, and not the former, of which the limitations and qualifications may then appear accidental and arbitrary. Thus, to take an illustration from the subject of Ethics, with which I am here primarily concerned, I may begin by laying down as a principle that "all pain of human or rational beings is to be avoided"; and then afterwards may be led to enunciate the wider rule that "all pain is to be avoided"; it being made evident to me that the difference of rationality between two species of sentient beings is no ground for establishing a fundamental ethical distinction between their respective pains. In this case I shall ultimately regard the wider rule as the principle, and the narrower as a deduction from it; in spite of my having been led by a process of reasoning from the latter to the former.

(Henry Sidgwick, "The Establishment of Ethical First Principles," Mind 4 [January 1879]: 106-11, at 106-7)