On September 27, 1983, stallman posted a message to the net.unix-wizards and net.usoft Usenet newsgroups with the subject line "new Unix implementation." The post announced his intention to write "a complete Unix-compatible software system called GNU (for Gnu's Not Unix) and give it away free to everyone who can use it." The announcement is the founding document of the gnu-project and the opening move of the free software movement as an organized effort.
The post was written in the first person singular and was entirely practical in its initial framing: Stallman was starting work on a free operating system and was asking for contributions of time, money, code, and equipment. The philosophical argument was present but compact: "I consider that the golden rule requires that if I like a program I must share it with other people who like it." The deeper elaboration of the ethical framework would come in the gnu-manifesto two years later.
The choice of Unix compatibility was strategic. Unix was the dominant technical computing environment of the early 1980s, its design was well-understood, it was not itself freely available (AT&T's licensing terms were restrictive), and building a compatible system would make the free alternative immediately usable to the large population of Unix users. The name "GNU" — a recursive acronym for "GNU's Not Unix" — signaled both the relationship to Unix and the independence from it.
The announcement was made while Stallman was still employed at the mit-ai-lab. He resigned from MIT employment in January 1984 — to avoid conflicts of interest over distributing GNU as free software — marking the start of the period documented in founding-gnu-and-fsf-1983-1991. The free-as-in-freedom-williams biography records Stallman as having weighed his options carefully before posting: he considered finding another programming job but concluded that building the free operating system was the only adequate response to the collapse of the hacker community he had witnessed.