Letter to Robert Applegarth (3 December 1869) 
Source:  The Abolition of Landed Property http://www.marxists.org/archive/marx/works/1869/12/03.htm (3 December 1869)
                                    
“When a theory is sufficiently general to cover many fields of application, it acquires some "truth" from each of them.”
            Methods of Mathematics Applied to Calculus, Probability, and Statistics (1985) 
Context: When a theory is sufficiently general to cover many fields of application, it acquires some "truth" from each of them. Thus... a positive value for generalization in mathematics.
        
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Richard Hamming 90
American mathematician and information theorist 1915–1998Related quotes
                                        
                                         Re: "Choose the Right Language" in "Tutorial" by Norvig and Pitman http://groups.google.com/group/comp.lang.lisp/msg/99d41ab4a42978b1 (Usenet article). 
Usenet articles, Miscellaneous
                                    
Life Without and Life Within (1859), The One In All
                                        
                                        Kenneth Boulding (1953) in letter to Bertalanffy, cited in: Ludwig von Bertalanffy (1968) General System Theory. p. 14 
1950s
                                    
                                        
                                        Preface (August, 1864) 
The Mechanical Theory of Heat (1867)
                                    
Source: Dynamics in Psychology, 1940, p. 55
Source: General System Theory (1968), 4. Advances in General Systems Theory, p. 80