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In the 1970s, when I was working on CLU, Alan Kay, Dan Ingalls, Adele Goldberg, Ted Kaehler, Diana Merry and Scott Wallace created the programming language Smalltalk. Smalltalk had classes and subclasses, which were introduced originally in the Simula 67 programming language. Around 1986, when I was focused on distributed systems, I was asked to give the keynote for OOPSLA, which is a conference focused on object-oriented programming, and I decided to look into what was going on with Smalltalk and its derivatives. While going through the literature, I discovered people were interested in something called type hierarchy, but they didnt understand exactly what it was. I came up with a way to define this, which I described in my talk at OOPSLA.

Several years later, in the early 1990s, I got an email asking if I could explain the definition of the Liskov substitution principle. That was the first time I heard that term because I had not introduced it that way, and I then discovered that there was a huge interest in this question. This interest was not surprising because type hierarchy is an important concept for object-oriented programs. A little while later, Jeannette M. Wing suggested that we could come up with a formal definition of the Liskov substitution principle. That was what led to the manuscript that came out in 1994.

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Laying the foundations of programming and system design - Nature.com