"Copyleft: Pragmatic Idealism" (~1998) develops the argument that copyleft is both a principled ethical position and a pragmatically sound strategy — that in this case the idealistic choice and the practical choice happen to coincide. The essay is a response to those who suggested that stallman chose copyleft out of ideological rigidity and that a more pragmatic approach would use permissive licenses.
stallman's argument is that using copyleft — implemented in gpl-v2 and related licenses — advances the practical goal of building a large, sustainable body of free software precisely because it prevents that software from being converted into proprietary software. Every free program that exists in perpetuity as free software benefits the community; every free program that gets reappropriated and locked down is a loss. copyleft maximizes the accumulation of permanently free software over time.
The essay also defends the idealistic dimension of the choice. stallman argues that there is value in acting on principle even when a pragmatic calculation might point in a different direction — and that in the case of copyleft, the principled choice and the pragmatic choice are aligned. This alignment is not accidental: copyleft was designed to serve the goal of software freedom, and its legal mechanism implements that goal efficiently.
The essay situates copyleft within the broader project of the gnu-project and free-software-foundation: building the infrastructure — legal, technical, and social — for a world in which software freedom is the norm rather than the exception. The gpl-copyleft-mechanism is not just a license but a tool for transforming the software ecosystem.
The argument connects to the contemporaneous open-source-definition-schism of 1998, which produced the open source movement's preference for permissive licenses as "business-friendly." stallman's essay implicitly responds to that preference by insisting that business-friendliness achieved at the cost of user freedom is not a goal worth pursuing. why-copyleft covers the same ground more briefly.