Before revision:  Near thermodynamic equilibrium, response is completely determined by the nature of spontaneous fluctuations, according to the fluctuation-dissipation theorem (FDT). This deep connection forms the basis of powerful experimental techniques, but also implies that highly-responsive equilibrium devices are always plagued by noise .
After revision:  Near thermodynamic equilibrium, response is completely determined by the nature of spontaneous fluctuations, according to the fluctuation-dissipation theorem provides a powerful theoretical and experimental tool to determine the nature of response by observing spontaneous equilibrium fluctuations .