Black box

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Black box systems
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This article is about the abstract concept of black box systems. For flight data recorders, see Flight recorder. For other uses, see Black box (disambiguation).

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, an algorithm, or the human brain.


To analyse something, as an open system, with a typical "black box approach", only the behavior of the stimulus/response will be accounted for, to infer the (unknown) box. The usual representation of this black box system is a data flow diagram centered in the box.

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 behavior of a black box. Many other engineers, scientists and epistemologists, as Mario Bunge,[6] used and perfected the black box theory in the 1960s.

System theory

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

The black box is an abstraction representing a class of concrete open system 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 the approach under consideration, which is purely external or phenomenological. In other words, only the behavior of the system will be accounted for.

— Bunge[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 I accidentally pushed the button marked “!”—the Box increased in temperature by 20 °C.
... ... Etc.
When the observer can also do some stimulus (input), the relation with the black box is not only an 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 hypotheses 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.

Other theories

Black box theories are things defined only in terms of their function.[9][10] The term black box theory is applied to any field, philosophy and science or otherwise where some inquiry or definition is made into the relations between the appearance of something (exterior/outside), i.e. here specifically the thing's black box state, related to its characteristics and behaviour within (interior/inner).[11][12]

Specifically, the inquiry is focused upon a thing that has no immediately apparent characteristics and therefore has only factors for consideration held within itself hidden from immediate observation. The observer is assumed ignorant in the first instance as the majority of available data is held in an inner situation away from facile investigations. The black box element of the definition is shown as being characterised by a system where observable elements enter a perhaps imaginary box with a set of different outputs emerging which are also observable.[13]

Adoption in humanities

In Humanities disciplines such as Philosophy of Mind and Behaviorism, one of the black box theories uses is as a method to describe/understand psychological factors in fields such as marketing where applied to an analyses of consumer behaviour.[14][15][16]

The black box theory of consciousness, which states that the mind is fully understood once the inputs and outputs are well defined,[17] and generally couples this with a radical skepticism regarding the possibility of ever successfully describing the underlying structure, mechanism, and dynamics of the mind.[clarification needed]


  • In computer programming and software engineering, black box testing is used to check that the output of a program is as expected, given certain inputs.[18] The term "black box" is used because the actual program being executed is not examined.
  • In computing in general, a black box program is one where the user cannot see the 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 is 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 mathematical modeling, a limiting case.
  • In philosophy and psychology, the school of behaviorism sees the human mind as a black box;[19] see other theories.
  • In neorealist international relations theory, the sovereign state is generally considered a black box: states are assumed to be unitary, rational, self-interested actors, and the actual decision-making processes of the state are disregarded as being largely irrelevant. Liberal and constructivist theorists often criticize neorealism for the "black box" model, and refer to much of their work on how states arrive at decisions as "breaking open the black box".
  • 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.

— Ashby[4]

(...) This simple rule proved very effective and is an illustration of how the Black Box principle in cybernetics can be used to control situations that, if gone into deeply, may seem very complex.
A further example of the Black Box principle is the treatment of mental patients. The human brain is certainly a Back Box, and while a great deal of neurological research is going on to understand the mechanism of the brain, progress in treatment is also being made by observing patients' responses to stimuli.

— Duckworth, Gear and Lockett[20]

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".[21]

See also


  1. Cauer, Wilhelm; Theorie der linearen Wechselstromschaltungen, Vol.I, Akademische Verlags-Gesellschaft Becker und Erler, Leipzig, 1941.
  2. Cauer, Emil; Mathis, Wolfgang; and Pauli, Rainer; "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, Vitold; "Summary of the history of circuit theory", Proceedings of the IRE, vol 50, Iss 5, pp. 848-855, May 1962.
  4. 4.0 4.1 4.2 4.3 Ashby, W. Ross; An introduction to cybernetics, London: Chapman & Hall, 1956, chapter 6: The black box, pp. 86–117.
  5. Wiener, Norbert; Cybernetics: or the Control and Communication in the Animal and the Machine, MIT Press, 1961, ISBN 0-262-73009-X, page xi
  6. 6.0 6.1 Bunge, Mario; "A general black-box theory", Philosophy of Science, Vol. 30, No. 4, 1963, pp. 346-358. jstor/186066
  7. Glanville, Ranulph; "Black Boxes", Cybernetics and Human Knowing, 2009, pp. 153-167.
  8. See for 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. Definition from
  10. definition from highbeam
  11. Black box theory applied briefly to Isaac Newton
  12. Usage of term
  13. Physics dept, Temple University, Philadelphia
  14. Institute for working futures part of Advanced Diploma in Logistics and Management. Retrieved 11/09/2011
  15. Black-box theory used to understand Consumer behaviour Marketing By Richard L. Sandhusen. Retrieved 11/09/2011
  16. designing of websites Retrieved 11/09/2011
  17. the Professor network
  18. Beizer, Boris; Black-Box Testing: Techniques for Functional Testing of Software and Systems, 1995, ISBN 0-471-12094-4
  19. "Mind as a Black Box: The Behaviorist Approach", pp. 85-88, in Friedenberg, Jay; and Silverman, Gordon; Cognitive Science: An Introduction to the Study of Mind, Sage Publications, 2006.
  20. WE Duckworth, AE Gear and AG Lockett (1977), "A Guide to Operational Research". DOI:10.1007/978-94-011-6910-3

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