I belive that both inputs and specifications have their intended purposes. In terms of actually constructing a device, it is impossible to do so based on solely inputs. Specifications are needed for the actual construction of the device and for the device to be constructed identically during the manufacturing process. However, inputs are important because they lay out how the design meets the intended needs without going into unnecessary detail. For instance, when describing a project to a group of investors, it is much more useful to talk to them about the inputs rather than the specifications. The inputs as the question of how the device works without the unnecessary details such as individual measurements or tolerances. Also, you need an input document in order to make a specifications document.