Garrison staffer Tom Bethel explained Garrison's belief in propinquity as follows:
"In Dallas, at the time of the assassination there lived a Russian-émigré oil geologist named George De Mohrenschildt who had befriended Lee Harvey Oswald after Lee returned from the Soviet Union in 1962 (whither he had defected in 1959). There was another member of the Dallas émigré community named George Bouhe, who knew De Mohrenschildt (who knew Oswald). And city directories showed Bouhe lived right opposite … Jack Ruby! (he shot Oswald, just in case you had forgotten.) And there you have the long-sought Oswald-Ruby link—based on propinquity.
If you start with a belief in a conspiracy and then "reverse engineer" the event you can find these connections - however tangential - between/among individuals that are related, directly or indirectly, to the assassination. Then you can claim that these connections show a conspiracy; and demand to be proven wrong. On one level, it's convincing; or at least plausible. But only if you start with the original conclusion of a conspiracy occurring. It's a kind of post hoc ergo propter hoc thinking with confirmation bias thrown in.
This type of thinking is what Garrison did all of time; and is what his acolytes also do (that's why they're acolytes after all). It's what Fonzi and others in that "first" HSCA investigation did as well.
It's why since Garrison the conspiracy "community" has been lost, adrift in this "the government killed JFK" sea of paranoia and fantasies. E.g, Oliver Stone, "JFK".