Boxed and obscured Tao

When a system is particularly complicated, complex or unknown, and we are only interested to know what happens from the entry (stimulus) to the output (effect), what we can do is to identify an input (IN) and output (OUT ):

and then put the system into a blackbox and no one matters what's going on inside, observing only the relation between IN and OUT:

The OUT/IN relation determined in each condition of interest characterizes the system completely.

The 99% of the systems normally used, from mobile phones to watches, are used as blackboxes, only the experts know what's inside and how they work...