In a language serving a complex (i.e., real) community,
it is absence of structured heterogeneity that would be dysfunctional
Uriel Weinreich wrote this in the much influential paper "Empirical foundations for a theory of language change" from 1968, and it is as true today as it was then. The paper was written by him, William Labov and Marvin Herzog. It was published and finalised after Weinreich's death, he died in 1967 at the age of 40. In the last weeks of his life, he devoted his time to discussing and writing this very paper. In the preface of the paper, his two co-authors write about him and the work behind this paper, and how honoured they are to have written this with him. They write that by section 2.4, the personal editing of the paper by Uriel Weinreich ends. While the finalisation of the paper was their work and the arguments represent their joint discussions, they underline that the conception and much of the writing was his.
The paper is well-cited for good reasons, they outline the necessity of seeing language as inherently diverse and as interactions in a social community, they critique the generative views of language that ignore variation. Their formulation of the "five problems" (constraints, transition, embedding, evolution and actuation) is still thought-provoking and important. If you are interested in language change and variation, you definitely should read it. In particular, if you read the 2009 paper by Levinson and Evans ("The myth of language universals") and found it rewarding - read this.
Besides being of great importance to the field, this paper also represent the last thoughts on this matter that we have from Uriel Weinreich. When reading the paper, I go from reading sections that he was able to personally write and edit, to sections where he could not do this any longer. It feels very personal and important. I, too, feel honoured to share his thoughts, and I hope that all of you will as well.
Evans, N. and Levinson, S. C. (2009). The myth of language universals: Language diversity and its importance for cognitive science. Behavioral and Brain Sciences, 32. Free PDF here.
Weinreich, U., Labov, W. & Herzog, M.I. 1968. Empirical foundations for a theory of language change. In Directions for Historical Linguistics: A Symposium , W. Lehmann & Y. Malkiel (eds), 97– 195. Austin TX: University of Texas Press. Free PDF here.