"Ultimately, most objects, man-made or not are 'black boxes.'"

OK, I see what you're getting at.

Three questions about black boxes:

1) Does the input have to be fully known/observable to constitute a black box? When investigating a population of neurons, we can give stimulus to these cells, but we cannot be sure that we are aware of all the inputs they are receiving. So we effectively do not entirely understand the input being given.

2) Does the output have to be fully known/observable to constitute a black box? When we measure the output of a population of neurons, we also cannot be sure of the totality of information being sent out, due to experimental limitations.

3) If one does not understand a system one uses, does that fact alone make that system a black box? In that case there are absolute black boxes, like the human mind, about which complete information *is not known*, and relative black boxes, like the car or TCP/IP, about which complete information *is not known to the current user*.

4) What degree of understanding is sufficient for something not to be called a black box?

Depending on how we answer these things, it will determine whether black box comes to mean:

1) Anything that is identifiable as a 'part', whose input and output is known but whose intermediate working/processing is not understood. 2) Anything that is identifiable as a 'part' whose input, output and/or processing is not understood. 3) Any 'part' that is not completely understood (i.e. presuming access to all information) 4) Anything that is not understood by the user at the time 5) Anything that is not FULLY understood by the user at the time.

We will quickly be in the realm where anything and everything on earth is considered to be a black box, if we take the latter definitions. So how can this word/metaphor be most profitably wielded?