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History of development and suppression

It is impossible to know with certainty what another person is thinking, making suppression difficult. The concept is developed throughout the Bible, most fully in the writings of Saul of Tarsus (e.g., "For why should my freedom be judged by another's conscience ?" 1 Corinthians 10:29).

Although Greek philosophers Plato and Socrates had discussed freedom of thought minimally, the edicts of King Ashoka (3rd century BC) have been called the first decree respecting freedom of conscience. In European tradition, aside from the decree of religious toleration by Constantine I at Milan in 313, the philosophers Themistius, Michel de Montaigne, Baruch Spinoza, John Locke, Voltaire, Alexandre Vinet, and John Stuart Mill and the theologians Roger Williams and Samuel Rutherford have been considered major proponents of the idea of freedom of conscience (or "soul liberty" in the words of Williams).

Queen Elizabeth I revoked a thought censorship law in the late sixteenth century, because, according to Sir Francis Bacon, she did "not to make windows into men's souls and secret thoughts". During her reign, however, a number of books published by theorist Giordano Bruno spurred controversy, mentioning topics banned by the Catholic Church such as the possibility of an infinite universe. Unwilling to recant these ideas, Bruno was eventually burned as a heretic in Rome by the Italian Inquisition, in turn becoming a martyr for free thought.

Oliver Cromwell is described by Ignaz von Döllinger as "the first among the mighty men of the world to set up one special religious principle, and to enforce it so far as in him lay:... The principle of liberty of conscience and the repudiation of religious coercion".

However, freedom of expression can be limited through censorship, arrests, book burning, or propaganda, and this tends to discourage freedom of thought. Examples of effective campaigns against freedom of expression are the Soviet suppression of genetics research in favor of a theory known as Lysenkoism, the book-burning campaigns of Nazi Germany, the radical anti-intellectualism enforced in Cambodia under Pol Pot and in Nazi Germany under Adolf Hitler, the strict limits on freedom of expression imposed by the Communist governments of the People's Republic of China and Cuba or by Capitalist dictatorships such as those of Augusto Pinochet in Chile and Francisco Franco in Spain.

The Sapir–Whorf hypothesis, which states that thought can be embedded in language, would support the claim that an effort to limit the use of words of language is actually a form of restricting freedom of thought. This was explored in George Orwell 's novel 1984, with the idea of Newspeak, a stripped-down form of the English language alleged to lack the capacity for metaphor and limiting expression of original ideas.

More recently, the development of neuroimaging technologies have raised concerns about entities being able to read and subsequently suppress thought. Although the issue is complicated by the mind-body problem, these concerns form the emerging field of neuroethics and neuroprivacy.