In the first half of 1963, Dijkstra described the execution of a program as a living “tree” that can grow “branches” and “twigs”. As the tree gets older — that is, as the execution of the program progresses — branches and twigs can fall off the tree, and others can grow in their place. Dijkstra used the metaphor of the living tree to unify three seemingly disparate topics: dynamic memory management, subroutine invocations, and multiprogramming.

John C. Reynolds wrote to me:

As Dijkstra was fond of pointing out, the community of people who call themselves “software engineers” is marred nowadays by an abundance of second rate work — so much so that others have come to disdain the term and call themselves “computer scientists”. But in the 60’s and 70’s, people such as Hoare, Wirth, and Dijkstra proudly and properly called themselves software engineers, and managed to be simultaneously rigorous and useful.

In early 1962, Edsger W. Dijkstra presented an analogy of a classroom teacher calling upon one of her pupils. By doing so, he conveyed some subtleties of “dynamic memory management” and “concurrent process behavior”.

In his 1972 `Notes on Structured Programming' [1, Chapter 1], Dijkstra introduced a methodology called step-wise program composition. He explained how to compose a program in minute steps, deciding each time as little as possible so that the correctness of each step is obvious.