His first and best known work, A Candle in the Dark: Or, A Treatise Concerning the Nature of Witches & Witchcraft, was used unsuccessfully by George Burroughs, a Baptist preacher, in his defense during the Salem witch trials. The second book, published in 1661, was A Perfect Discovery of Witches, named in honour of Reginald Scot's Discoverie of Witchcraft, the first book of its kind in the English language; his third was The Doctrine of Devils (1676).
A Candle in the Dark
In A Candle in the Dark, Ady attacked current ideas of witchcraft by arguing (as did his opponents) directly from the Bible. In the first of the book's three parts, Ady argues that the well-known prohibition against witches in Deuteronomy 18:10-11
- There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch. Or a charmer, or a consulter with familiar spirits, or a wizard, or a necromancer. ([Authorized King James Version])
should be read to define witches as "the popish (Catholic) rout, the contrivers of charms to delude the people. (emphasis in original). The second part questions why contemporary proofs of witchcraft do not have biblical support.
- Where is it written in all the Old and New Testaments that a witch is a murderer, or hath power to kill by witchcraft, or to afflict with any disease or infirmity? Where is it written that witches have biggs (nipples) for imps to suck on ... that the devil setteth privy marks upon witches ... that witches can hurt corn or cattle ... or can fly in the air .... Where do we read of a he-devil or she-devil, called incubus or succubus, that useth generation or copulation? 
The third part attacks contemporary writes on witchcraft and demonology. Ady suggests the book Daemonologie attributed to King James was ghostwritten by the Bishop of Winchester. He also disagrees strongly with Thomas Cooper ("a bloody persecutor of the poor"), author of the book The Mystery of Witchcraft (1617) and with Matthew Perkins's Discourse (1608), calling it "a collection of mingled notions" from Jean Bodin, Bartolommeo Spina, and "other popish blood suckers" who wrote "great volumes of horrible lies and impossibilities."  Ady also corrects John Gaule (author of Select Cases of Conscience touching Witches and Witchcrafts (1646) and Mysmatia, the Mag-astromancer (1652)) and George Gifford (author of A Discourse of the Subtle Practices of Devils by Witches and Sorcerers (1587) and A Dialogue Concerning Witches and Witchcrafts (1593)).
The scholar and librarian George Lincoln Burr called A Candle in the Dark "one of the bravest and most rational of the early protests".
I will speak of one man ... that went about in King James his time ... who called himself, the Kings Majesties most excellent Hocus Pocus, and so was called, because that at the playing of every Trick, he used to say, Hocus pocus, tontus tabantus, vade celeriter jubeo, a dark composure of words, to blinde the eyes of the beholders, to make his Trick pass the more currently without discovery (Thomas Ady, "A Candle in the Dark", 1655).
This quote from "A Candle in the Dark" was used an epigraph to Dan Chapman's 2014 novel, "Closed Circuit".
Thomas Ady's works are directly influenced by Reginald Scot and his Discoverie of Witchcraft. They are also influenced by works such as Hocus Pocus Junior on juggling and stage magic. Works of the period sometimes used specific examples of illusionist tricks to reveal superstition.
Ady's works resemble other works on freedom of conscience written at that time, particularly Roger Williams's well-known The Bloudy Tenant of Persecution for the Cause of Conscience, written in 1644. Like Ady, Williams makes extensive use of scripture to show why religious persecution is wrong.
His book also is one of the earliest references to the origin of the word hocus pocus as a Latin-like phrase used by a conjurer to distract his audience from his sleight of hand, which also relates to where the word hoax comes from. See the articles Hocus Pocus (magic) and hoax on Wikipedia for more information, as well as the etymologies for hoax and hocus pocus.
Ady's books here, or Reginald Scot's and Roger William's works cited earlier, resemble books on conscience that came later that also use the Bible, notably those of the Christian Abolitionist Movement. For instance, A Condensed Anti-Slavery Bible Argument (1845) by George Bourne, and God Against Slavery (1857) by George B. Cheever.
Some biographical information about Ady is independent of his writings. On 10 June 1634 Thomas Ady or Adye of Weathersfield, "A famous Dr of Physick", married Barbara the daughter of William Sparrow of Sible Hedingham. Of Ady's father-in-law, it is said in the history of Essex (1831) that "William Sparrow, of Sible Hedingham, the eldest surviving son, succeeded his father, who died in 1589: he married Joan, daughter of John Finch, of Gestingthorp, by whom he had three sons, John, William, and Joseph, and two daughters, Jame and Barbara; the last of whom was married to Thomas Ady, M.D. of Wethersfield. William, the second son, was a clothier, father of William, attorney-at-law, of Sible Hedingham, and died in 1648.".
They lived in Wethersfield, and their son was educated at Felsted and Sidney Sussex College, Cambridge; and was admitted to Grey's Inn in 1667. Records show that Barbara was baptised on 9 September 1610, and Thomas left a PCC will [PROB 11/339] dated 15 October 1662 and proved 20 May 1672, in which he describes himself as "being a professed member of the true Christian Protestant Church of England desireing to live and dye in the true Christian faith". He named his wife Barbara, daughter Dorothy married to William Collard, son Thomas under 21 years, and daughters Joana and Barbara in his will. His daughter Barbara married Mark Mott, who died and was buried in Wethersfield 22 May 1694.
- Ady, Thomas (1656). A Candle in the Dark, or a treatise concerning the nature of witches and witchcraft: being advice to the judges, sheriffs, justices of the peace and grandjurymen what to do before they pass sentence on secuh as are arraigned for their lives as witches.
- George Knowles, "Thomas Ady", accessed 7 January 2007.
- In the Devil's Snare: The Salem Witchcraft Crisis of 1692, p. 251, by Mary Beth Norton
- Cooper, Thomas (1617). The Mystery of Witchcraft: Discovering the Truth, Nature, Occasions, Growth and Power Thereof.
- Perkins, Matthew (1608). A Discourse of the Damned Art of Witchcraft; so far forth as it is revealed in the Scriptures, and manifest by true experience.
- Chapman, D. (2014), Closed Circuit, UK: Concept, ISBN 978-1499191615
- Ady, Tomas (1634). Hocus Pocus Junior.
- I. Opie and P. Opie, The Oxford Dictionary of Nursery Rhymes (Oxford: Oxford University Press, 1951, 2nd edn., 1997), p. 303.
- Online Etymology Dictionary: Hoax, http://www.etymonline.com/index.php?term=hoax, accessed 13 March 2011.
- Online Etymology Dictionary: Hocus,http://www.etymonline.com/index.php?search=hocus, accessed 13 March 2011.
- Bourne, George., A Condensed Anti-Slavery Bible Argument, 1845
- Cheever, George B., God Against Slavery, 1857
- Burke's Landed Gentry (1952), ""
- Philip Morant, The History and Antiquities of the County of Essex (1768), p. 289
- Berry's Essex Pedigrees found in "The visitations of Essex by Hawley, 1552; Hervey, 1558; Cooke, 1570; Raven, 1612; and Owen and Lilly, 1634". To which are added an appendix containing Berry's Essex pedigrees, Part 2 (p 715), ""
- The history and topography of the county of Essex, comprising its ancient and modern history. A general view of its physical character, productions, agricultural condition, statistics &c. &c (1831) by Thomas Wright and W. Bartlett, ""
- Old Conjuring Books by Trevor H. Hall, 1972
- Philip Morant's The History and Antiquities of the County of Essex (1768), p376
- Candle in the Dark, *A Perfect Discovery of Witches and *The Doctrine of Devils – full text of Ady's books.
- Biography by George Knowles.
- Reaction to The Discovery of Witchcraft by Stephen Forrester.
- The Advent of Modern Thought in Popular Literature from The Cambridge History of English and American Literature.
- Discussion on the JREF forum.