“A class is a set of objects that share a common structure and a common behavior”

Sociology and modern systems theory (1967)

The Art of Persuasion
Context: As to the objection that these rules are common in the world, that it is necessary to define every thing and to prove every thing, and that logicians themselves have placed them among their art, I would that the thing were true and that it were so well known... But so little is this the case, that, geometricians alone excepted, who are so few in number that they are a single in a whole nation and long periods of time, we see no others that know it.

Source: "刘延东：中南人民守望相助　友谊历久弥坚" https://www.mfa.gov.cn/ce/cezanew//chn/zngxss/jyjl/t1457724.htm (27 April 2017)

Source: Liberalism (1911), Chapter III, The Movement Of Theory, p. 30.

Source: 1860s, Evidence as to Man's Place in Nature (1863), Ch.1, p. 34

Keynote address, Democratic National Convention, New York (12 July 1976). (see External links)

Book XIX, sec. 59.
Naturalis Historia

Source: Object-oriented design: With Applications, (1991), p. 142

Source: The Heritage Universe, Summertide (1990), Chapter 5, “Summertide Minus Thirty” (p. 61)