Recently Alex Acosta asked me a question: "What is the function of the doorbell? Make sounds. However, the doorbells are created to inform people that someone is waiting at the door of your home."
I answered that the function of a door bell is to inform people that someone is waiting at the door of your home BY making sound.
His question highlights the difficulty of defining the function of a technical system when it is assumed that the function of a technical system is only what it should do, but not how. Such an assumption is only correct with respect to how function is implemented but not with respect to how system interacts with others. The latter how is part of its function.
That is why how a door bell generates a sound is not part of its function. But how a door bell interacts with other actors/users is part of its function. And it interacts by making sound.
The function to inform people that someone is waiting at the door of your home is too abstract. It is applicable to a range of devices, not door bells only. For example, it is applicable to devices that inform people by flashes of light, or by producing smell, etc. Thus, if we want to define the function of door bells only we have to specify how it informs people. To this end we specify that it informs people by making sounds. We don't have to specify how a door bell actually makes sounds. This is another how, internal to the system. We only have to specify external how, how the system interacts with other actors.
The latter how is still for a range of door bells, not for a specific door bell only. We have to further specify it by indicating what kind of sound it generates, its amplitude, frequencies, etc. to narrow it down to a specific door bell.
But as soon as we start specifying the how, the what starts getting blurred. A device that informs people by making sounds can be used not only as a door bell but for many other purposes. The more specific the how is the wider the area of possible applications of a device. If we only specify what, then there is an infinite range of how it could be to achieved. But if we only specify how, there is an infinite range of what can be achieved with the help of this how. Thus, the uncertainty principle holds:
ΔWhat × ΔHow ≥ const