In science, computing, and engineering, a black box is a system 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). The term can be used to refer to many inner workings, such as those of a transistor, an engine, an algorithm, the human brain, or an institution or government.
To analyze 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 meaning of the 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, such as Mario Bunge, used and perfected the black box theory in the 1960s.
In systems theory, 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 understanding of a black box is based on the "explanatory principle", the hypothesis of a causal relation between the input and the output. This principle states that input and output are distinct, that the system has observable (and relatable) inputs and outputs and that the system is black to the observer (non-openable).[6]
An observer makes observations over time. All observations of inputs and outputs of a black box can be written in a table, in which, at each of a sequence of times, the states of the box's various parts, input and output, are recorded. Thus, using an example from Ashby, examining a box that has fallen from a flying saucer might lead to this protocol:
| Time | States of input and output | |
|---|---|---|
| 11:18 | I did nothing—the Box emitted a steady hum at 240 Hz. | |
| 11:19 | I pushed over the switch marked K: the note rose to 480 Hz and remained steady. | |
| 11:20 | I accidentally pushed the button marked “!”—the Box increased in temperature by 20 °C. | |
| ... | Etc. |
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 active feedback in the box/observer relation, promoting what in control theory is called a feed forward architecture.
The modeling process is the construction of a predictive mathematical model, using existing historic data (observation table).
A developed black box model is a validated model when black-box testing methods[7] ensures that it is, based solely on observable elements.
With back testing, out of time data is always used when testing the black box model. Data has to be written down before it is pulled for black box inputs.
Black box theories are those theories defined only in terms of their function.[8] [9] The term can be applied in any field where some inquiry is made into the relations between aspects of the appearance of a system (exterior of the black box), with no attempt made to explain why those relations should exist (interior of the black box). In this context, Newton's theory of gravitation can be described as a black box theory.[10]
Specifically, the inquiry is focused upon a system 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.[11]
In humanities disciplines such as philosophy of mind and behaviorism, one of the uses of black box theory is to describe and understand psychological factors in fields such as marketing when applied to an analysis of consumer behaviour.[12] [13] [14]
Black Box theory is even wider in application than professional studies: