It is said that a good metaphor frees the mind . . . then imprisons it. The metaphor of business as a model for the nonprofit sector seems to be tracing this proverbial path. Over the past decade, many principles borrowed from corporate management, venture capital, entrepreneurship, and investment portfolios have been grafted onto the work of foundations and nonprofits. These new ideas have stimulated a considerable amount of innovation, bringing to the field venture philanthropy funds, social entrepreneurs, capacity building grants and the increasingly common goal of taking small nonprofit organizations to scale. It is true that certain basic principles of strategy, expertise and efficiency apply to the nonprofit sector, just as they do to every other kind of enterprise. But these broad principles need to be thought through carefully in the specialized context of the nonprofit sector if they are to be usefully applied. We may already be reaching the point where the business metaphor has imprisoned much new thinking in the social sector -- and the limitations are most apparent in this widespread ambition, shared by many funders and nonprofit leaders alike, of "going to scale" through rapid organizational growth.