One of the core concepts in sustainable development is that technology can be used to assist people to meet their developmental needs. Technology to meet these sustainable development needs is often referred to as appropriate technology, which is an ideological movement (and its manifestations) originally articulated as intermediate technology by the economist E. F. Schumacher in his influential work "Small Is Beautiful" and now covers a wide range of technologies. Both Schumacher and many modern-day proponents of appropriate technology also emphasise the technology as people-centered. Today appropriate technology is often developed using open source principles, which have led to open-source appropriate technology (OSAT) and thus many of the plans of the technology can be freely found on the Internet. OSAT has been proposed as a new model of enabling innovation for sustainable development. Free Software, including that which runs Wikimedia, is licensed under the GNU GPL, or a similar license. It is this licensing model that has enabled the global wiki movement, not the licensing model of Open Source, which by itself is inadequate to prevent proprietary vendors from redistributing unfree versions of designs and software that they obtained gratis. Richard Stallman, the founder of the movement that created the GNU/Linux operating system, has explained in detail the ethical and legal underpinnings of these projects as Four Essential Freedoms of Free Software.