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Black box

This article is about black box systems. For other uses, see Black box (disambiguation).
Scheme of a black box. Only the behavior of the stimulus/response will be accounted for, to infer the (unknowned) box.

In science, computing, and engineering, a black box is a device, system or object which can be viewed in terms of its inputs and outputs (or transfer characteristics), without any knowledge of its internal workings. Its implementation is "opaque" (black). Almost anything might be referred to as a black box: a transistor, algorithm, or the human brain.

The opposite of a black box is a system where the inner components or logic are available for inspection, which is most commonly referred to as a white box (sometimes also known as a "clear box" or a "glass box").


The modern term "black box" seems to have entered the English language around 1945. In electronic circuit theory the process of network synthesis from transfer functions, which led to electronic circuits being regarded as "black boxes" characterized by their response to signals applied to their ports, can be traced to Wilhelm Cauer who published his ideas in their most developed form in 1941.[1] Although Cauer did not himself use the term, others who followed him certainly did describe the method as black-box analysis.[2] Vitold Belevitch[3] puts the concept of black-boxes even earlier, attributing the explicit use of two-port networks as black boxes to Franz Breisig in 1921 and argues that 2-terminal components were implicitly treated as black-boxes before that.

In cybernetics, a full treatment was given by Ross Ashby in 1956.[4] A black box was described by Norbert Wiener in 1961 as an unknown system that was to be identified using the techniques of system identification.[5] He saw the first step in self-organization as being to be able to copy the output behaviour of a black box.


The open system theory have the foundations of the black box theory. Both have focus on input and output flows, representing exchanges with its surroundings.

The black box is an abstraction representing a class of concrete open systems which can be viewed solely in terms of its "stimuli inputs" and "output reactions": "The constitution and structure of the box are altogether irrelevant to approach under consideration, which is purely external or phenomenological. In other words, only the behavior of the system will be accounted for."[6]

The understanding of a black box is based on the "explanatory principle", the hypothesis of a causal relation between the input and the output, and:[7]

  • input and output being believed to be distinct,
  • having observable (and relatable) inputs and outputs,
  • being black to the observer (non-openable).

Recording of observed states

The observed hydrograph is a graphic of the response of a watershed (a blackbox) with its runoff (red) to an input of rainfall (blue).

An observer makes observations over time. All observations of inputs and outputs of a black box can be written in a table with the form:

↓Time States of input and output
... ... ...
... ... ...

in which, at each of a sequence of times, the states of the box’s various parts, input and output, are recorded. Thus, using the Ashby's example, the box that fell from the Flying saucer might lead to the protocol:[4]

↓Time States of input and output
11:18 a.m. I did nothing—the Box emitted a steady hum at 240 c/s.
11:19 I pushed over the switch marked K: the note rose to 480 c/s and remained steady.
11.20 Accidentally I pushed the button marked “!”—the Box increased in temperature by 20°C.
... ... Etc.
When observer can also do some stimulus (input), the relation with the black box is not only a observation, but an experiment.

Thus every system, fundamentally, is investigated by the collection of a long protocol, drawn out in time, showing the sequence of input and output states. From this there follows the fundamental deduction that all knowledge obtainable from a Black Box (of given input and output) is such as can be obtained by re-coding the protocol (the observation table); all that, and nothing more.[4]

If the observer also controls input, the investigation turns into an experiment (illustration), and hypothesis about cause-and-effect can be tested directly.

When the "experimenter" is also motivated to "control the box", there is an active feedback in the box/observer relation, promoting what in control theory is a feed forward architecture.


The modeling process is the construction of a predictive mathematical model, using existing historic data (observation table).

Testing black box model

A developed black box model is a validated model when black-box testing methods[8] ensures that, based solely on observable elements.

See also Backtesting: inputs for past events (not used in the "modeling effort") are entered into the model to see how well the output matches the known results.


  • In computing in general, a black box program is one where the user cannot see its inner workings (perhaps because it is a closed source program) or one which has no side effects and the function of which need not be examined, a routine suitable for re-use.
  • Also in computing, a black box refers to a piece of equipment provided by a vendor, for the purpose of using that vendor's product. It is often the case that the vendor maintains and supports this equipment, and the company receiving the black box typically are hands-off.
  • In neural networking or heuristic algorithms (computer terms generally used to describe 'learning' computers or 'AI simulations') a black box is used to describe the constantly changing section of the program environment which cannot easily be tested by the programmers. This is also called a white box in the context that the program code can be seen, but the code is so complex that it is functionally equivalent to a black box.
  • In physics, a black box is a system whose internal structure is unknown, or need not be considered for a particular purpose.
  • In cryptography to capture the notion of knowledge obtained by an algorithm through the execution of a cryptographic protocol such as a zero-knowledge proof protocol. If the output of the algorithm when interacting with the protocol can be simulated by a simulator that interacts only the algorithm, this means that the algorithm 'cannot know' anything more than the input of the simulator. If the simulator can only interact with the algorithm in a black box way, we speak of a black box simulator.

Black Box theory is, however, even wider in application than these professional studies:

The child who tries to open a door has to manipulate the handle (the input) so as to produce the desired movement at the latch (the output); and he has to learn how to control the one by the other without being able to see the internal mechanism that links them. In our daily lives we are confronted at every turn with systems whose internal mechanisms are not fully open to inspection, and which must be treated by the methods appropriate to the Black Box.


Other uses of the term

  • In aviation, the flight recorder is often called a "black box", especially by the media, although it is usually bright orange to facilitate its discovery after a crash. In an airplane or helicopter, the flight recorder consists of two independent devices that are usually combined in a single unit. The flight data recorder logs information about controls and sensors, and the cockpit voice recorder preserves the most recent sounds in the cockpit, including conversation of the pilots. If an accident happens, investigators can use the recordings to assist in the investigation.
  • In amateur radio the term "black box operator" is a disparaging or self-deprecating description of someone who operates factory made radios without having a good understanding of how they work. Such operators do not build their own equipment (an activity called "homebrewing") or repair their own "black boxes".[11]

See also


  1. ^ W. Cauer. Theorie der linearen Wechselstromschaltungen, Vol.I. Akad. Verlags-Gesellschaft Becker und Erler, Leipzig, 1941.
  2. ^ E. Cauer, W. Mathis, and R. Pauli, "Life and Work of Wilhelm Cauer (1900 – 1945)", Proceedings of the Fourteenth International Symposium of Mathematical Theory of Networks and Systems (MTNS2000), p4, Perpignan, June, 2000. Retrieved online 19 September 2008.
  3. ^ Belevitch, V, "Summary of the history of circuit theory", Proceedings of the IRE, vol 50, Iss 5, pp848-855, May 1962.
  4. ^ a b c Ashby, W. Ross 1956. An introduction to cybernetics. London: Chapman & Hall, chapter 6 The black box, p86–117.
  5. ^ Wiener, Norbert 1961. Cybernetics: or the Control and Communication in the Animal and the Machine. page xi, MIT Press. ISBN 0-262-73009-X
  6. ^ Mario Bunge (1963), "A general black-box theory". Philosophy of Science. Vol. 30. No. 4, pp. 346-358. jstor/186066
  7. ^ R. Glanville (2009), "Black Boxes", Cybernetics and Human Knowing, pp153-167
  8. ^ See ex. the British standard BS 7925-2 (Software component testing), or its 2001 work draft,
    BCS SIGIST (British Computer Society Specialist Interest Group in Software Testing), "Standard for Software Component Testing", Working Draft 3.4, 27. April 2001. webpage.
  9. ^ Black-Box Testing: Techniques for Functional Testing of Software and Systems, by Boris Beizer, 1995. ISBN 0-471-12094-4
  10. ^ "Mind as a Black Box: The Behaviorist Approach", pp 85-88, in Cognitive Science: An Introduction to the Study of Mind, by Jay Friedenberg, Gordon Silverman, Sage Publications, 2006
  11. ^

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