"The Establishment of Ethical First Principles" (1879), in Essays on Ethics and Method (Oxford: Clarendon Press, 2000), p. 30 https://books.google.it/books?id=Ci9x5WY3NesC&pg=PA30
Works

The Methods of Ethics
Henry SidgwickFamous Henry Sidgwick Quotes
The Methods of Ethics (1884), p. 381
Elements of Politics (3rd ed., 1908), Ch. 1: Scope and Method of Politics
Source: The Methods of Ethics (1874), Book 3, chapter 13, section 3 (7th ed., 1907)
Source: The Methods of Ethics (1874), Book 4, chapter 5, section 3 (7th ed., 1907)