Thought experiment

Thought experiment

A thought experiment (from the German "Gedankenexperiment") is a proposal for an experiment that would test a hypothesis or theory but cannot actually be performed due to practical limitations; instead its purpose is to explore the potential consequences of the principle in question. Famous examples of thought experiments include Schroedinger's cat, illustrating quantum indeterminacy through the manipulation of a perfectly sealed environment and a single radioactive atom, and Maxwell's demon, in which a supernatural being is instructed to attempt to violate the second law of thermodynamics.


As the contemporary philosopher Martin Cohen puts it, "much of modern physics is built not upon measurement but on thought experimentation". [Cohen, Martin, "Wittgenstein's Beetle and Other Classic Thought Experiments", Blackwell, (Oxford), 2005, p5.] As Cohen argues, the Renaissance period and the Enlightenment were characterized by breakthroughs in ways of seeing the world, not merely by new methods (and tools) for 'measuring' it.

Thus it is that perhaps the key experiment in the history of modern science, again toppling the lofty but inaccurate view of Aristotle, is Galileo's demonstration that falling objects must fall at the same rate regardless of their masses. This is widely thought [Cohen, Martin, "Wittgenstein's Beetle and Other Classic Thought Experiments", Blackwell, (Oxford), 2005, p55-56.] to have been a straightforward physical demonstration, involving climbing up the Leaning Tower of Pisa and dropping two heavy weights off it, whereas in fact, it was clearly a logical demonstration, using the 'thought experiment' technique. The 'experiment' is described by Galileo in "Discorsi e dimostrazioni matematiche" (1628) (literally, 'Mathematical Discourses and Demonstrations') thus:

"Salviati". If then we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower, and the slower will be somewhat hastened by the swifter. Do you not agree with me in this opinion?

"Simplicio". You are unquestionably right.

"Salviati". But if this is true, and if a large stone moves with a speed of, say, eight while a smaller moves with a speed of four, then when they are united, the system will move with a speed less than eight; but the two stones when tied together make a stone larger than that which before moved with a speed of eight. Hence the heavier body moves with less speed than the lighter; an effect which is contrary to your supposition. Thus you see how, from your assumption that the heavier body moves more rapidly than ' the lighter one, I infer that the heavier body moves more slowly.

cite web |url= |title=Galileo on Aristotle and Acceleration |accessdate=2008-05-24 |

Although the extract does not convey the elegance and power of the 'demonstration' terribly well, it is clear that it is a 'thought' experiment, rather than a practical one. Strange then, as Cohen says, that philosophers and scientists alike refuse to acknowledge either Galileo in particular, or the thought experiment technique in general for its pivotal role in both science and philosophy.

Instead, many philosophers prefer to consider 'Thought Experiments' to be merely the use of a hypothetical scenario to help understand the way things actually are.

Different kinds of thought experiments

There are many different kinds of thought experiments. All thought experiments, however, employ a methodology that is a priori, rather than empirical, in that they do not proceed by observation or physical experiment.

If a "Gedankenexperiment" is going to be utilized, it is imperative to formulate all data into a truth table, where you ask yourself a question and if the answer is yes, than, you can ask yourself another question, and so on. Though if the question you pose to yourself is "no", then one must go back to field a "yes" answer. All Gedankenexperiments revolve around truth tables, and a Gedankenexperiment is an introspective analysis of the truth you're trying to answer; a Gedankenexperiment is not meant to be shared with anybody.Clarifyme|date=October 2008

Thought experiments have been used in a variety of fields, including philosophy, law, physics, and mathematics. In philosophy, they have been used at least since classical antiquity, some pre-dating Socrates. In law, they were well-known to Roman lawyers quoted in the Digest. In physics and other sciences, notable thought experiments date from the 19th and especially the 20th century, but examples can be found at least as early as Galileo.

Origins and use of the literal term "thought experiment"

Witt-Hansen established that Hans Christian Ørsted was the first to use the Latin-German mixed term "Gedankenexperiment" (lit. experiment conducted in the thoughts) circa 1812. Ørsted was also the first to use its entirely German equivalent, "Gedankenversuch", in 1820.

Much later, Ernst Mach used the term "Gedankenexperiment" in a different way to exclusively denote the "imaginary" conduct of a "real" experiment that would be subsequently performed as a "real physical experiment" by his students (thus the contrast between physical and mental experimentation) with Mach asking his students to provide him with explanations whenever it happened that the results from their subsequent, real, physical experiment had differed from those of their prior, imaginary experiment.

The English term "thought experiment" was coined (as a calque) from Mach's "Gedankenexperiment", and it first appeared in the 1897 English translation of one of Mach’s papers. Prior to its emergence, the activity of posing hypothetical questions that employed subjunctive reasoning had existed for a very long time (for both scientists and philosophers). However, people had no way of categorizing it or speaking about it. This helps to explain the extremely wide and diverse range of the application of the term "thought experiment" once it had been introduced into English.

Thought experimentation in general

In its broadest usage, thought experimentation is the process of employing imaginary situations to help us understand the way things really are (or, in the case of Herman Kahn’s "scenarios", understand something about something in the future). The understanding comes through reflection upon this imaginary situation. Thought experimentation is an a priori, rather than an empirical process, in that the experiments are conducted within the imagination (i.e., Brown’s (1993) "laboratory of the mind"), and never in fact.

Thought experiments, which are well-structured, well-defined hypothetical questions that employ subjunctive reasoning (irrealis moods) -- "What might happen (or, what might have happened) if . . . " -- have been used to pose questions in philosophy at least since Greek antiquity, some pre-dating Socrates (see Rescher). In physics and other sciences many famous thought experiments date from the 19th and especially the 20th Century, but examples can be found at least as early as Galileo.

Thought experiments have been used in philosophy, physics, and other fields (such as cognitive psychology, history, political science, economics, social psychology, law, organizational studies, marketing, and epidemiology). In law, the synonym "hypothetical" is frequently used for such experiments.

Regardless of their intended goal, all thought experiments display a patterned way of thinking that is designed to allow us to explain, predict and control events in a better and more productive way.

The theoretical consequences of thought experimentation

In terms of their theoretical consequences, thought experiments generally:
* challenge (or, even, refute) a prevailing theory, often involving the device known as reductio ad absurdum,
* confirm a prevailing theory,
* establish a new theory, or
* simultaneously refute a prevailing theory and establish a new theory through a process of mutual exclusion.

The practical application of thought experimentation

Thought experiments often introduce interesting, important and valuable new perspectives on old mysteries and old questions; yet, although they may make old questions irrelevant, they may also create new questions that are not easy to answer.

In terms of their practical application, thought experiments are generally created in order to:
* challenge the prevailing status quo (which includes activities such as correcting misinformation (or misapprehension), identify flaws in the argument(s) presented, to preserve (for the long-term) objectively established fact, and to refute specific assertions that some particular thing is permissible, forbidden, known, believed, possible, or necessary);
* extrapolate beyond (or interpolate within) the boundaries of already established fact;
* predict and forecast the (otherwise) indefinite and unknowable future;
* explain the past;
* the retrodiction, postdiction and postcasting of the (otherwise) indefinite and unknowable past;
* facilitate decision making, choice and strategy selection;
* solve problems, and generate ideas;
* move current (often insoluble) problems into another, more helpful and more productive problem space (e.g., see functional fixedness);
* attribute causation, preventability, blame and responsibility for specific outcomes;
* assess culpability and compensatory damages in social and legal contexts;
* ensure the repeat of past success; or
* examine the extent to which past events might have occurred differently.
* ensure the (future) avoidance of past failures.

Thought experimentation in science

Scientists tend to use thought experiments in the form of imaginary, "proxy" experiments which they conduct prior to a real, "physical" experiment (Ernst Mach always argued that these "gedankenexperiments" were "a necessary precondition for physical experiment"). In these cases, the result of the "proxy" experiment will often be so clear that there will be no need to conduct a physical experiment at all.

Scientists also use thought experiments when particular physical experiments are impossible to conduct (Carl Gustav Hempel labeled these sorts of experiment "theoretical experiments-in-imagination"), such as Einstein's thought experiment of chasing a light beam, leading to Special Relativity. This is a unique use of a scientific thought experiment, in that it was never carried out, but led to a successful theory, proven by other empirical means.

Causal reasoning in thought experiments

Generally speaking, there are seven types of thought experiments in which one reasons from causes to effects, or effects to causes:

Prefactual thought experiments

"Prefactual (before the fact) thought experiments" speculate on possible future outcomes, given the present, and ask "What will be the outcome if event E occurs?"

Counterfactual thought experiments

"Counterfactual (?contrary to established fact?) thought experiments" speculate on the possible outcomes of a different past; and ask "What might have happened if A had happened instead of B?" (e.g., "If Isaac Newton and Gottfried Leibniz "had" cooperated with each other, what would mathematics look like today?").

emifactual thought experiments

"Semifactual thought experiments" speculate on the extent to which things might have remained the same, despite there being a different past; and asks the question Even though X happened instead of E, would Y have still occurred? (e.g., Even if the goalie "had" moved left, rather than right, could he have intercepted a ball that was traveling at such a speed?).

Semifactual speculations are an important part of clinical medicine.

Prediction, forecasting and nowcasting

The activities of "prediction", "forecasting" and "nowcasting" attempt to project the circumstances of the present into the future (the only difference between these identically patterned activities being the distance of their speculated future from the present).


The activity of "hindcasting" involves running a forecast model after an event has happened in order to test whether the model's simulation is valid.

Retrodiction (or postdiction)

The activity of "retrodiction" (or "postdiction") involves moving backwards in time, step-by-step, in as many stages as are considered necessary, from the present into the speculated past, in order to establish the ultimate cause of a specific event (e.g., Reverse engineering and Forensics).


The activity of "backcasting" involves establishing the description of a very definite and very specific future situation. It then involves an imaginary moving backwards in time, step-by-step, in as many stages as are considered necessary, from the future to the present, in order to reveal the mechanism through which that particular specified future could be attained from the present.

It is important to recognize that a major difficulty with all types of thought experiment, and particularly with "counterfactual" thought experiments, is that there are no formally accepted criteria for accurately measuring the risk of either "Type I errors" (False positive) or "Type II errors" (False negative) in the choice of a potential causative factor.

Thought experiments in philosophy

Whereas thought experiments in physics are intended to give us "a priori" knowledge of the natural world, philosophy attempts to produce "a priori" knowledge of our concepts::" [Philosophical and scientific] investigations differ both in their methods (the former is a priori, and the latter a posteriori) and in the metaphysical status of their results (the former yields facts that are metaphysically necessary and the latter yields facts that are metaphysically contingent). Yet the two types of investigations resemble each other in that both, if successful, uncover new facts, and these facts, although expressed in language, are generally not about language (except for investigations in such specialized areas as philosophy of language and empirical linguistics)." [Ackermann, F., "Philosophical Knowledge", pp.342-345 in Dancy, J. & Sosa, E. (eds.), "A Companion to Epistemology", Blackwell Publishers, (Oxford), 1993, pp.342-343.]

In philosophy, a thought experiment typically presents an imagined scenario with the intention of eliciting an intuitive response about the way things are in the thought experiment. (Philosophers might also supplement their thought experiments with theoretical reasoning designed to support the desired intuitive response.) The scenario will typically be designed to target a particular philosophical notion, such as morality, or the nature of the mind or linguistic reference. The intuitive response to the imagined scenario is supposed to tell us about the nature of that notion in any scenario, real or imagined.

For example, a thought experiment might present a situation in which an agent intentionally kills an innocent for the benefit of others. Here, the relevant question is whether the action is moral or not, but more broadly whether a moral theory is correct that says morality is determined solely by an action?s consequences (See Consequentialism). John Searle imagines a man in a locked room who receives written sentences in Chinese, and returns written sentences in Chinese, according to a sophisticated instruction manual. Here, the relevant question is not whether or not the man understands Chinese, but more broadly, whether a functionalist theory of mind is correct.

It is generally hoped that there is universal agreement about the intuitions that a thought experiment elicits. (Hence, in assessing their own thought experiments, philosophers may appeal to "what we should say," or some such locution.) A successful thought experiment will be one in which intuitions about it are widely shared. But often, philosophers differ in their intuitions about the scenario.

Other philosophical uses of imagined scenarios arguably are thought experiments also. In one use of scenarios, philosophers might imagine persons in a particular situation (maybe ourselves), and ask what they would do.

For example, John Rawls asks us to imagine a group of persons in a situation where they know nothing about themselves, and are charged with devising a social or political organization (See the veil of ignorance). The use of the state of nature to imagine the origins of government, as by Thomas Hobbes and John Locke, may also be considered a thought experiment. Similarly, Nietzsche, in On the Genealogy of Morals, speculated about the historical development of Judeo-Christian morality, with the intent of questioning its legitimacy.

One of the earliest known thought experiments was Avicenna's "Floating Man" thought experiment in the 11th century. He asked his readers to imagine themselves suspended in the air isolated from all sensations in order to demonstrate human self-awareness and self-consciousness, and the substantiality of the soul. [Seyyed Hossein Nasr and Oliver Leaman (1996), "History of Islamic Philosophy", p. 315, Routledge, ISBN 0415131596.]

Philosophical thought experiments and possibility

The scenario presented in a thought experiment must be possible in some sense. In many thought experiments, the scenario would be nomologically possible, or possible according to the laws of nature. John Searle's Chinese Room is nomologically possible.

Some thought experiments present scenarios that are not nomologically possible. In his Twin Earth thought experiment, Hilary Putnam asks us to imagine a scenario in which there is a substance with all of the observable properties of water (e.g., taste, color, boiling point), but which is chemically different from water. It has been argued that this thought experiment is not nomologically possible, although it may be possible in some other sense, such as metaphysical possibility. It is debatable whether the nomological impossibility of a thought experiment renders intuitions about it moot.

In some cases, the hypothetical scenario might be considered metaphysically impossible, or impossible in any sense at all. David Chalmers says that we can imagine that there are zombies, or persons who are physically identical to us in every way but who lack consciousness. This is supposed to show that physicalism is false. However, some argue that zombies are inconceivable: we can no more imagine a zombie than we can imagine that 1+1=3.

Other criticisms of philosophical thought experiments

The use of thought experiments in philosophy has received other criticisms, especially in the philosophy of mind. Daniel Dennett has derisively referred to certain types of thought experiments such as the Chinese Room experiment as "intuition pumps", claiming they are simply thinly veiled appeals to intuition which fail when carefully analyzed. Another criticism that has been voiced is that some science fiction-type thought experiments are too wild to yield clear intuitions, or that any resulting intuitions could not possibly pertain to the real world. Another criticism is that philosophers have used thought experiments (and other a priori methods) in areas where empirical science should be the primary method of discovery, as for example, with issues about the mind.

Famous thought experiments


Thought experiments are popular in physics and include:
* Newton's cannonball (Newton's laws of motion)
* Brownian ratchet (Richard Feynman's "perpetual motion" machine that does not violate the second law and does no work)
* Casimir cones (basis for almost perpetual motion machine fueled by entropy) []
* Galileo's ship (classical relativity principle) 1632
* Galileo's Leaning Tower of Pisa experiment (rebuttal of Aristotelian Gravity)
* GHZ experiment (quantum mechanics)
* Kepler's Dream (change of point of view as support for the Copernican hypothesis)
* EPR paradox (quantum mechanics) (forms of this have actually been performed)
* Ladder paradox (special relativity)
* Maxwell's demon (thermodynamics) 1871
* Quantum suicide (quantum mechanics)
* Schrödinger's cat (quantum mechanics)
* Twin paradox (special relativity)
* Wigner's friend (quantum mechanics)
* Wittgenstein's rod (engineering mechanics) – an exercise in visualization
* Bucket argument – argues that space is absolute, not relational
* Airplane - Treadmill Conundrum


The field of philosophy makes extensive use of thought experiments:
* Big Book (ethics)
* Brain-in-a-vat (epistemology, philosophy of mind)
* Brainstorm machine
* Changing places (reflexive monism, philosophy of mind)
* China brain (physicalism, philosophy of mind)
* Chinese room (philosophy of mind, artificial intelligence, cognitive science)
* Coherence (philosophical gambling strategy)
* God's Debris (religion and awareness)
* Hilary Putnam's Twin Earth thought experiment in the philosophy of language and philosophy of mind
* How many men? (taxation as theft)
* Inverted spectrum
* Mary's room (philosophy of mind)
* Molyneux's Problem (admittedly, this oscillated between empirical and a-priori assessment)
* Original position (politics)
* Philosophical zombie (philosophy of mind, artificial intelligence, cognitive science)
* Plank of Carneades
* Social contract theories
* The Ship of Theseus (concept of identity)
* Simulated reality (philosophy, computer science, cognitive science)
* Strawson's auditory world
* Swamp man (personal identity, philosophy of mind)
* Shoemaker's "Time Without Change" (metaphysics)
* Trolley problem (ethics)
* The Violinist (ethics)
* Zeno's paradoxes (classical Greek problems of the infinite)
* Robinson's rotating/still spheres (metaphysics)


* Balls and vase problem (infinity and cardinality)
* Gabriel's Horn (infinity)


* Braitenberg vehicles (robotics, neural control and sensing systems) (some have actually been built)
* Doomsday argument (anthropic principle)
* Infinite monkey theorem (probability, infinity)
* Halting problem (limits of computability)
* The Lady, or the Tiger? (human nature) [While the problem presented in this short story's scenario is not unique, it is extremely unusual. Most thought experiments are intentionally (or, even, sometimes unintentionally) skewed towards the inevitable production of a particular solution to the problem posed; and this happens because of the way that the problem and the scenario are framed in the first place. In the case of "The Lady, or the Tiger?", the way that the story unfolds is so "end-neutral" that, at the finish, there is no "correct" solution to the problem. Therefore, all that one can do is to offer one's own innermost thoughts on how the account of human nature that has been presented might unfold ? according to one's own experience of human nature ? which is, obviously, the purpose of the entire exercise. The extent to which the story can provoke such an extremely wide range of (otherwise equipollent) predictions of the participants' subsequent behaviour is one of the reasons the story has been so popular over time.]
* Turing machine (limits of computability)
* Dining Philosophers (computer science)

ee also

* Alternate history (fiction)
* Counterfactual conditional
* Forecast
* Futures studies
* Futures techniques
* Hindcast
* Hypothesis
* Irrealis moods
* "Mapping" (see entry under conceptual metaphor)
* "Nearly possible worlds" (see under Possible world)
* "Nowcasting" (see entry under weather forecasting)
* Possible world
* Post hoc ergo propter hoc
* Postdiction
* Prediction
* Prognosis
* Retrodiction
* Scenario planning
* Scenario test
* Subjunctive mood


ignificant articles about thought experiments or thought experimentation

* Dennett, D.C., "Intuition Pumps", pp.180-197 in Brockman, J., "The Third Culture: Beyond the Scientific Revolution", Simon & Schuster, (New York), 1995. []
* Galton, F., "Statistics of Mental Imagery", "Mind", Vol.5, No.19, (July 1880), pp.301-318.
* Hempel, C.G., "Typological Methods in the Natural and Social Sciences", pp.155-171 in Hempel, C.G. (ed.), "Aspects of Scientific Explanation and Other Essays in the Philosophy of Science", The Free Press, (New York), 1965.
* Kuhn, T. "A Function for Thought Experiements", in "The Essential Tension" (Chicago: University of Chicago Press, 1979), pp. 240--
* Mach, E., "On Thought Experiments", pp.134-147 in Mach, E., "Knowledge and Error: Sketches on the Psychology of Enquiry", D. Reidel Publishing Co., (Dordrecht), 1976. [Translation of "Erkenntnis und Irrtum" (5th edition, 1926.] .
* Popper, K., "On the Use and Misuse of Imaginary Experiments, Especially in Quantum Theory", pp.442-456, in Popper, K., "The Logic of Scientific Discovery", Harper Torchbooks, (New York), 1968.
* Rescher, N., "Thought Experiment in Pre-Socratic Philosophy", pp.31-41 in Horowitz, T. & Massey, G.J. (eds.), [ "Thought Experiments in Science and Philosophy",] Rowman & Littlefield, (Savage), 1991.
* Witt-Hansen, J., "H.C. Ørsted, Immanuel Kant and the Thought Experiment", "Danish Yearbook of Philosophy", Vol.13, (1976), pp.48-65.
* Jacques, V., Wu, E., Grosshans, F., Treussart, F., Grangier, P. Aspect, A., & Roch, J. (2007). Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment, "Science, 315", p. 966-968. []

Books about thought experiments

* Brown, J.R., "The Laboratory of the Mind: Thought Experiments in the Natural Sciences", Routledge, (London), 1993.
* Browning, K.A. (ed.), "Nowcasting", Academic Press, (London), 1982.
* Cohen, Martin, "Wittgenstein's Beetle and Other Classic Thought Experiments", Blackwell (Oxford) 2005
* Cohnitz, D., "Gedankenexperimente in der Philosophie", Mentis Publ., (Paderborn, Germany), 2006.
* Craik, K.J.W., "The Nature of Explanation", Cambridge University Press, (Cambridge), 1943.
* Cushing, J.T., "Philosophical Concepts in Physics: The Historical Relation Between Philosophy and Scientific Theories", Cambridge University Press, (Cambridge), 1998.
* DePaul, M. & Ramsey, W. (eds.), "Rethinking Intuition: The Psychology of Intuition and Its Role in Philosophical Inquiry", Rowman & Littlefield Publishers, (Lanham), 1998.
* Gendler, T.S., "Thought Experiment: On the Powers and Limits of Imaginary Cases", Garland, (New York), 2000.
* Gendler, T.S. & Hawthorne, J., "Conceivability and Possibility", Oxford University Press, (Oxford), 2002.
* H?ggqvist, S., "Thought Experiments in Philosophy", Almqvist & Wiksell International, (Stockholm), 1996.
* Hanson, N.R., "Patterns of Discovery: An Inquiry into the Conceptual Foundations of Science", Cambridge University Press, (Cambridge), 1962.
* Harper, W.L., Stalnaker, R. & Pearce, G. (eds.), "Ifs: Conditionals, Belief, Decision, Chance, and Time", D. Reidel Publishing Co., (Dordrecht), 1981.
* Hesse, M.B., "Models and Analogies in Science", Sheed and Ward, (London), 1963.
* Holyoak, K.J. & Thagard, P., "Mental Leaps: Analogy in Creative Thought", A Bradford Book, The MIT Press, (Cambridge), 1995.
* Horowitz, T. & Massey, G.J. (eds.), [ "Thought Experiments in Science and Philosophy",] Rowman & Littlefield, (Savage), 1991.
* Kahn, H., "Thinking About the Unthinkable", Discus Books, (New York), 1971.
* Kuhne, U., "Die Methode des Gedankenexperiments", Suhrkamp Publ., (Frankfurt/M, Germany), 2005.
* Leatherdale, W.H., "The Role of Analogy, Model and Metaphor in Science", North-Holland Publishing Company, (Amsterdam), 1974.
* Roese, N.J. & Olson, J.M. (eds.), "What Might Have Been: The Social Psychology of Counterfactual Thinking", Lawrence Erlbaum Associates, (Mahwah), 1995.
* Shanks, N. (ed.), "Idealization IX: Idealization in Contemporary Physics (Poznan Studies in the Philosophy of the Sciences and the Humanities, Volume 63)", Rodopi, (Amsterdam), 1998.
* Shick, T. & Vaugn, L., "Doing Philosophy: An Introduction through Thought Experiments (Second Edition)", McGraw Hill, (New York), 2003.
* Sorensen, R.A., "Thought Experiments", Oxford University Press, (Oxford), 1992.
* Tetlock, P.E. & Belkin, A. (eds.), "Counterfactual Thought Experiments in World Politics", Princeton University Press, (Princeton), 1996.
* Thomson, J.J. {Parent, W. (ed.)}, "Rights, Restitution, and Risks: Essays in Moral Theory", Harvard University Press, (Cambridge), 1986 .
* Vosniadou, S. & Ortony. A. (eds.), "Similarity and Analogical Reasoning", Cambridge University Press, (Cambridge), 1989.
* Wilkes, K.V., "Real People: Personal Identity without Thought Experiments", Oxford University Press, (Oxford), 1988.

External links

* [] "Philosophy Bites" podcast: Nigel Warburton interviews Julian Baggini on Thought Experiments
* [ Stevinus, Galileo, and Thought Experiments] Short essay by S. Abbas Raza of [ "3 Quarks Daily"]
* [ Thought experiment generator] , an entertaining visual aid to running your own thought experiment
* Articles on [ "Thought Experiments"] in the "PhilSci Archive", an electronic archive for preprints in the philosophy of science.
* The [ Clean Slate Society Forums] "If we could start again, from a clean slate, knowing what we know today, what would we do differently?"

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