Von Neumann languages do not have useful properties for reasoning about programs. Axiomatic and denotational semantics are precise tools for describing and understanding conventional programs, but they only talk about them and cannot alter their ungainly properties. Unlike von Neumann languages, the language of ordinary algebra is suitable both for stating its laws and for transforming an equation into its solution, all within the "language."
"Can Programming Be Liberated From the von Neumann Style?". John Backus' 1977 Turing Award lecture, published in "Communications of the ACM", Volume 21, No. 8 (pp. 639-640), August 1978.