Structure and Interpretation of Computer Programs

The Hidden Origins of Lisp

Beginnings are important. They may not fully dictate the trajectory of their antecedents, yet it does seem they do have a profound impact on the character of their effects. For the human observer, beginnings are also a source of inspiration: good beginnings lend a strength of purpose, the possibility of greater good. The story of Lisp has a good beginning -- several of them, in fact -- closely tied to the theories of numbers, mathematical logic, functions and types as well as that of computing itself.

At their root, the histories of programming languages spring from, on one hand, the practical considerations of engineering and developer experience, and on the other hand, the principle of computability. This, in turn, ultimately traces its beginnings to the fundamental concepts of arithmetic and mathematical logic: what are numbers and how to we define them rigorously? These questions were asked and considered -- sometimes from a fairly vague philosophical perspective -- by great minds such as Leibniz (later 1600s; drafts published posthumously), Boole (1847), Grassmann (1861), Peirce (1881), Frege (1884), and Dedekind (1888). It was the Italian mathematician Giuseppe Peano, though, who in 1889 finally identified and distilled the essence of these explorations in terms that were more precisely formulated than those of his peers or intellectual forebearers. These were subsequently elaborated by successive generations of mathematicians prior to the advent of "high-level" programming languages in the 1950s.

Histories are complicated; complete ones are impossible and readable ones are necessarily limited and lacking in details. In our particular case, there is a complex lineage of mathematics leading to Lisp. However, for the sake of clarity and due to this being a preface and not a book in its own right, the mathematical and computational history leading to Lisp has been greatly simplified below. The four dominant historical figures discussed provide distinct insights and represent corresponding themes as mathematics evolved unwittingly toward a support for computing. Due to the limitation of scope, however, it might be better to view these as archetypes of mathematical discovery rather than historical figures one might come to know when reading a full history. Of the many themes one could discern and extract from these great minds, we focus on the following:

• Understanding and defining the underpinnings of arithmetic and logic ("What are numbers? What is counting?")
• Attempting to formally unify all of mathematics in a consistent framework of logic ("Can I express all of math in discrete logical assertions and statements?")
• Formally defining algorithms and computability ("Is there a procedure that can take any precise mathematical statement and decide whether the statement is true or false?")
• Creating the means by which symbolic computation and artificial reasoning could be made manifest ("Can we make machines solve problems that are usually considered to require intelligence?")¹

Each major topic above depended -- in one form or another -- upon the preceding topic, and the four famous mathematicans listed below embodied each of these themes. Small excerpts from their lives and work are shared as believed to have impacted the course of events that lead to Lisp's inception.