Priestley, Joseph (1733–1804)

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Joseph Priestley, the English scientist, nonconformist minister, educator, and philosopher, was born at Birstall, Yorkshire, the son of a cloth dresser. His mother died in 1740, and in 1742 Priestley was adopted by a childless well-to-do aunt, Mrs. Keighley, a convinced but unbigoted Calvinist. A sensitive child, Priestley suffered greatly because he could not convince himself that he had experienced the "new birth" essential, on the Calvinist scheme, for his salvation. As a result of these childhood miseries Priestley was left, he tells us, with "a peculiar sense of the value of rational principles of religion" as opposed to the "ignorance and darkness" of Calvinism.

Until the age of sixteen Priestley was educated at a conventional grammar school. For the next three years, his health being too poor for regular studies, he in large part educated himself, reading his way into mathematics, physics, and philosophy and undertaking the study of European and Middle Eastern languages. In 1752 his health improved and he entered Daventry Academy, a university-type institution set up by nonconformists because Oxford and Cambridge would not admit nonconformists to a degree.

At Daventry the emphasis was on free discussion, and the curriculum was considerably broader than at Oxford or Cambridge. Priestley was introduced to David Hartley's Observations on Man (1749) and was at onceand permanentlyconverted to Hartley's general outlook. The simplicity and generality of Hartley's associationist psychology appealed to Priestley's maturing scientific instincts; it provided a theoretical foundation for his belief in perfectibility through education; and it offered a psychological alternative to the doctrine of free will, which Priestley's reading of Anthony Collins's Philosophical Inquiry concerning Human Liberty and Necessity (1714) had already caused him to reject.

In 1755 Priestley entered the ministry, taking over a decaying congregation at Needham Market, Suffolk. Stammering and unorthodox, he was not a success as a minister. He moved in 1758 to a more sympathetic but equally impoverished congregation at Nantwich in Cheshire. In an attempt to increase his income he set up a school where, perhaps the first to do so, he taught experimental science with the help of an "electrical machine" and an air pump.

Appointed in 1761 as "tutor of the languages" at Warrington Academy in Lancashire, Priestley taught oratory, literary criticism, grammar, history, and law, as well as languages. Characteristically, on all these latter topics Priestley developed ideas that he sooner or later published. The Rudiments of English Grammar (1761), many times reprinted, is typical of his innovating boldness, insofar as he tried to simplify English syntax by removing from it the complications introduced by classically trained grammarians. His A Chart of Biography (1765) and A New Chart of History (1769) were even more enthusiastically received; they won for him not only his sole academic distinction, the doctorate of laws of the University of Edinburgh, but also his fellowship of the Royal Society.

Priestley's days of relative isolation were now over. In 1762 he married an ironmaster's daughter, Mary Wilkinson, an intelligent woman with a sense of humor and considerable force of characterqualities she was to need in the years to come. His duties at Warrington left him free to visit London for a month each year, where he came into contact with an active group of scientists, philosophers, and political thinkers, including Benjamin Franklin and Richard Price. Franklin encouraged Priestley's project of writing a history of electrical experiments. The work that resulted, The History and Present State of Electricity, with original Experiments (1767), is a notable contribution to the history of science. Describing a number of important original experiments, it is also in some respects the most theoretically adventurous of Priestley's scientific works. It contains as well Priestley's reflections on the use of hypotheses in scientific procedures as a guide to experimentation.

Education and Government

Like many of his fellow dissenters, Priestley was greatly interested in educational reform. Education had, he thought, thus far concentrated unduly on the needs of the clergy. His An Essay on a Course of Liberal Education for Civil and Active Life (1765) is a plea for a curriculum that should be suitable for men of affairs, emphasizing history and public administration rather than the classical languages. Priestley did much to encourage the teaching of history in the nonconformist academies. A set of lectures that he delivered at Warrington (published in 1788 as Lectures on History and General Policy ) provided not only the academies but also the new American colleges with a text suitable for their needs; it was, indeed, recommended even at Cambridge. It is a summary account of the main historical sources, with an emphasis on commerce, law, and administration, rather than a historical textbook of the ordinary kind.

Priestley's political theory was closely related to his interest in education and his experience as a member of a minority group. In an appendix to his Essay on a Course of Liberal Education he developed an argument against the introduction of a state system of education, which would inevitably, he thought, favor the status quo and produce a quite undesirable uniformity of conduct and opinion. Like John Stuart Mill after him, Priestley gloried in diversity; uniformity, he said, is "the characteristic of the brute creation."

These reflections were more fully worked out in An Essay on the First Principles of Government (1768), which bears the subtitle On the Nature of Political, Civil and Religious Liberty. For Priestley, the preservation of civil liberty was the crucial political issue. Deciding who should participate in governmentwho, that is, should possess political, as distinct from civil, libertywas, he thought, a practical matter, to be settled by considering what groups in the community are most likely, if they possess political power, to act for the greatest happiness of the greatest number. Such groups remain entitled to power only as long as they continue so to act. Legislation, on Priestley's view, should be kept to the minimum. What that minimum is cannot be determined a priori but only as a result of political experiment. But we can see at once, Priestley thought, that legislation that restricts civil and religious liberty is bound to be against the interests of the community. Unlike most nonconformist upholders of toleration and unlike his master John Locke, Priestley was uncompromising on this point; he upheld unbounded liberty of expression even to atheists and Roman Catholics.

In Priestley's eyes, the noblest of occupations was that of the clergyman, not the lecturer, and in 1767 he accepted a call to Mill Hill, Leeds, a congregation to whom his religious views were exceptionally congenial. The years Priestley spent at Mill Hill were extremely important in his development; his salary, although small, sufficed for his needs, and his duties left him considerable leisure.


Priestley had long before abandoned both the doctrine of the atonement, on which he wrote critically in The Scripture Doctrine of Remission (1761), and orthodox Trinitarianism. Now he took what was to be the final step in his transition from Calvinism to Unitarianism. Christ, he argued, although the Messiah, was a man, and not even a perfect man. Priestley's subsequent theological writings were in large part an attempt to provemost maturely in his History of Early Opinions concerning Jesus Christ (1786)that Unitarianism was the doctrine of the early church. He defended his unorthodoxies both against clerical attack, as in his Letters to Dr. Horsley (17831786), and, as in his Letters to a Philosophical Unbeliever (Pt. I, 1780; Pt. II, 1787), against those who, like Edward Gibbon, could not understand why Priestley did not make a complete break with Christianity. Priestley valued his theological writings above all his other work. A firm belief in Providence is everywhere evidenced in his writings. Few men have committed themselves so often and so absolutely to the doctrine that "all is for the best in the best of all possible worlds," although he also believed that the future world couldand therefore wouldbe better.

Scientific Achievement

It was as a scientist that Priestley won his international reputation. He published in 1772 what was intended to be the second section of a general history of science, The History of the Present State of the Discoveries relating to Vision, Light and Colours ; but this work, invaluable though it still is to historians of science, did not arouse a great deal of interest. Priestley therefore abandoned his large-scale historical project and concentrated instead on chemistry. His first chemical publication, in 1772, was of an unusually practical character: It described a method of producing "mephitic julep," or soda water. But it was the paper "On Different Kinds of Air," which he read in that same year to the Royal Society," that at once established his reputation as a chemist. In 1774 he prepared the first edition of Experiments and Observations on Different Kinds of Air ; this he republished in a series of editions, with important changes in contents, in method of organization, and even in title, until 1790.

By the end of that period Priestley, following up the work of Joseph Black and Henry Cavendish, had considerably enlarged our knowledge of the chemical properties of gases. He differentiated between nine gases, of which only three had previously been known to science, and described a method of collecting them. Of particular importance was his preparation of "dephlogisticated air" (oxygen), which he produced on August 1, 1774, by heating red mercuric oxide. It then became clear that air was not an element. Priestley went on to examine the properties of oxygen; in a series of chemicobiological experiments he brought out its importance for animal life.

As a resourceful experimenter, using simple and economical methods, Priestley has had few equals. But it was left to others, to Cavendish and Antoine Lavoisier, to appreciate the theoretical significance of his work. Priestley had isolated oxygen and had observed its importance in combustion; he had passed a spark through a mixture of hydrogen and oxygen and had noticed that dew was formed. Yet his last scientific work (1800) bore the title The Doctrine of Phlogiston established and that of the Composition of Water refuted. Although he had himself carried out important quantitative experiments, he did not appreciate the significance of the quantitative considerations by which Lavoisier overthrew the phlogiston theory.


Much of Priestley's most important scientific work was carried out at Shelburne, where from 1772 until 1780 he acted as "librarian and literary companion" to the Earl of Shelburne. During these same years Priestley embarked upon his most substantial metaphysical works. He began in 1774 with An Examination of Dr. Reid's Inquiry into the Human Mind on the Principles of Commonsense, Dr. Beattie's Essay on the Nature and Immutability of Truth, and Dr. Oswald's Appeal to Commonsense on Behalf of Religion, commonly referred to as An Examination of the Scotch Philosophers. This is a vigorous polemic, which sets out to demonstrate the superiority of Hartley's psychology to the philosophy of the Scottish commonsense school, a philosophy that Priestley thought obviously reactionary insofar as it substituted for the simple Locke-Hartley theory of mind "such a number of independent, arbitrary, instinctive principles that the very enumeration of them is really tiresome." All the so-called instinctive beliefs of common sense can, Priestley set out to show, be derived from the operations of associative principles working on the materials provided by sensation. He came to regret in later life the tone of this publication but never its doctrines.


Hoping to make Hartley's views better known, Priestley published an abridged version of Hartley's Observations on Man in 1775 as Hartley's Theory of the Human Mind on the Principle of the Association of Ideas. In his preface, Priestley somewhat tentatively suggested that all the powers of the mind might derive from the structure of the brain. Even as a suggestion this created a considerable uproar, but Priestley was not to be intimidated by clerical clamor. Convinced that materialism was the natural metaphysical concomitant of Hartley's associative psychology, he set out, therefore, in his Disquisitions Relating to Matter and Spirit (1777) to demonstrate that materialism was theologically, scientifically, and metaphysically superior to orthodox dualism.

On the theological side, materialism had commonly been objected to on the ground that it is incompatible with immortality. Man, Priestley replied, is not "naturally" immortal; he is immortal only because, as we know from revelation, God chooses to resurrect him; this resurrection is of the body and therefore also of the body's mental powers. As for the commonplace metaphysical objections to materialism, these are based, according to Priestley, upon an untenable conception of matter as being by nature inert and therefore incapable of exerting mental activity. To such a concept of matter Priestley opposed the physical theories of his friend and fellow scientist John Michell and the Jesuit mathematician Roger Boscovich. Material objects, on their view, are centers of force; if this is the nature of matter, Priestley argued, there is no good reason for denying that mental operations are part of the activity of a material object. On the other hand, there are very good reasons for objecting to the traditional dualism, which is quite incapable of explaining how mind and body can enter into any sort of relationship.


Priestley had been a determinist long before he became a materialist, but not until 1777, in The Doctrine of Philosophical Necessity Illustrated, did he fully present his case against free will; indeed, even then he thought of himself as supplementing Thomas Hobbes, Collins, David Hume, and Hartley with illustrations rather than as working out an entirely independent position. The doctrine of free will, he argued, is theologically objectionable because it cannot be reconciled with the existence of an all-seeing Providence; from a metaphysical standpoint, it makes human actions quite unintelligible, and ethics has no need of it. As a basis for our everyday moral judgments, the distinction between acting voluntarily and acting under compulsion is certainly important, but this distinction does not, according to Priestley, rest upon a metaphysical conception of free will.

Priestley's metaphysical unorthodoxies considerably disturbed his old friends, provoking a candid but good-tempered correspondence with Richard Price, published in 1778 as A Free Discussion of the Doctrines of Materialism and Philosophical Necessity Illustrated. Developing his views on the relation between moral judgments and determinism, Priestley admitted that the determinist will prefer to avoid describing people as blameworthy or praiseworthy. He will say of them, rather, that they have acted, or have not acted, from good principlesfrom principles, that is, that are conducive to the general happiness. But the determinist's different method of describing moral conduct has, Priestley thought, no practical consequences, and if determinism is in some respects inconsistent with everyday usage, this is even more true of libertarianism.

Later Years

There was a real risk, however, that Priestley's reputation for materialism might endanger the earl of Shelburne's political ambitions. Perhaps for this reason Priestley and Shelburne parted amicably in 1780, when Priestley, refusing Shelburne's offer of a post in Ireland, took up residence in Birmingham. There he had a circle of congenial friends who were prepared to offer him financial as well as intellectual support. He became a member of the Lunar Society, with which were associated men of the caliber of Erasmus Darwin and James Watt, and he enjoyed the friendship and help of the scientifically minded potter Josiah Wedgwood, who supplied him with apparatus specifically designed for his chemical experiments. Much of Priestley's scientific work in this period, under Alessandro Volta's influence, conjoined his two main scientific interests: electricity and gases. He examined the effect of passing electrical sparks through a variety of gases and studied their thermal conductivity.

He was by no means unsympathetic to the laissez-faire sociopolitical attitude of Birmingham industrialists. In Some Considerations on the State of the Poor in General (1787) he strongly criticized the poor laws and elsewhere opposed apprenticeship laws and laws for regulating interest rates. On his view, any sort of social welfare legislation "debased the very nature of man" by treating him as someone who had to be provided for. Although Priestley warmly supported schemes for cooperative insurance against hardship, he was opposed to any legislation that might diminish independence or increase the power of the state over individuals.

political radicalism

In general terms, Priestley's life at Birmingham was a continuation and development of his earlier activities; theological controversy continued to be his main interest. But one event transformed his life and modified his political attitudes: the French Revolution. Reacting to that revolution, the British government became steadily more intolerant and conservative, and Priestley came to think that extensive political innovations were a necessary condition for the preservation of civil liberty. He moved toward political radicalism of the nineteenth-century kind in his Letters to Edmund Burke occasioned by his Reflections on the Revolution in France (1791) and in the anonymously published A Political Dialogue on the General Principles of Government (1791). He had formerly been accustomed to describe himself as "a Unitarian in religion but a Trinitarian in politics" because he had accepted the view that liberty rested on the balance between king, Commons, and House of Lords. He now came to feel that there should be but one source of political power, the will of the people as it would be represented in a reformed House of Commons.

On July 14, 1791, the Friends of the Revolution organized a dinner at Birmingham (Priestley was not present) in order to commemorate the fall of the Bastille. This was the last straw. With the encouragement, it would seem, of the authorities, an angry mob attacked the nonconformist chapels, then turned their attention to Priestley's house, destroying his books and furniture. Priestley was persuaded by his friends to leave Birmingham for London where he was, however, shunned by his scientific colleagues.

life in america

For some years, Priestley had been contemplating migration to the United States, where his three sons had already gone. In 1794 he left for New York and finally settled in Northumberland, Pennsylvania. There, still supported by his old friends, he continued to experiment and to write, mainly on theological questions.

He was disappointed, however, by the orthodoxy of the American clergy and alarmed by the growth of intolerance in the United States. Although he took no part in politics, he wrote an uncompromising exposition of his political and religious views in Letters to the Inhabitants of Northumberland (1799). There was talk of his being deported under the Aliens Act, but John Adams would not permit the application of the act to "poor Priestley." With the election of Thomas Jefferson to the presidency, Priestley was not only secure but also at last on good terms with authority. Jefferson consulted him on educational questions, and Priestley's Socrates and Jesus Compared (1803) precipitated Jefferson's "Syllabus" of his religious beliefs. Another of Priestley's works, The Doctrines of Heathen Religion Compared with those of Revelation (1804), awoke in Adams an enthusiasm for comparative religion. Priestley's last years, from 1801 until his death, were marred by ill health and bereavements, but his diversified intellectual interests remained with him until the end.

See also Boscovich, Roger Joseph; Collins, Anthony; Darwin, Erasmus; Determinism, A Historical Survey; Determinism and Freedom; Franklin, Benjamin; Hartley, David; Hobbes, Thomas; Hume, David; Jefferson, Thomas; Libertarianism; Locke, John; Materialism; Mill, John Stuart; Price, Richard.


Priestley's scientific writings have never been collected. The standard edition of his philosophical, theological, and miscellaneous writings is The Theological and Miscellaneous Works of Joseph Priestley, edited by John Towill Rutt, 25 vols. in 26 (London: G. Smallfield, 18171832). John Arthur Passmore, Joseph Priestley (New York, 1965) includes a short list of Priestley's writings, an introductory essay, and selections from his major works.

See also Thomas Henry Huxley, "Joseph Priestley," in Science and Culture (London, 1881); Thomas Edward Thorpe, Joseph Priestley (New York: Dutton, 1906); Edgar Fahs Smith, Priestley in America, 17941804 (Philadelphia: P. Blakiston's Son, 1920); Wallace Ruddell Aykroyd, Three Philosophers: Lavoisier, Priestley and Cavendish (London: Heinemann, 1935); Arthur Handley Lincoln, Some Political and Social Ideas of English Dissent, 17631800 (Cambridge, U.K., 1938); Francis Edward Mineka, The Dissidence of Dissent (Chapel Hill: University of North Carolina Press, 1944); Stephen Edelston Toulmin, "Crucial Experiments: Priestley and Lavoisier," in Journal of the History of Ideas 18 (2) (1957): 205220; and the detailed bibliographies in the Dictionary of National Biography and the Dictionary of American Biography.

John Passmore (1967)

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Priestley, Joseph (1733–1804)

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