The first search algorithm based on user behavior was invented more than 60 years ago. I learned this from a seminal paper authored by Yehoshua Bar-Hillel. Bar-Hillel was an influential logician, philosopher, and linguist. He was also known as a proponent of the categorial grammar. Being a logician, Bar-Hillel was very critical of statistical and insufficiently rigorous methods. So, he wrote opinionatedly:

A colleague of mine, a well-known expert in information theory, proposed recently, as a useful tool for literature search, the compiling of pair-lists of documents that are requested together by users of libraries. He even suggested, if I understood him rightly, that the frequency of such co-requests might conceivably serve as an indicator of the degree of relatedness of the topics treated in these documents. I believe that this proposal should be treated with the greatest reserve.

On one hand, Bar-Hillel was very critical. On the other hand, he was also politic and cited his friend invention anonymously. This left us wondering: Who was that prominent information theorist?