Candide satirises various philosophical and religious theories that Voltaire had previously criticised. Primary among these is Leibnizian optimism (sometimes called Panglossianism after its fictional proponent), which Voltaire ridicules with descriptions of seemingly endless calamity. Voltaire demonstrates a variety of irredeemable evils in the world, leading many critics to contend that Voltaire's treatment of evil—specifically the theological problem of its existence—is the focus of the work. Heavily referenced in the text are the Lisbon earthquake, disease, and the sinking of ships in storms. Also, war, thievery, and murder—evils of human design—are explored as extensively in Candide as are environmental ills. Bottiglia notes Voltaire is "comprehensive" in his enumeration of the world's evils. He is unrelenting in attacking Leibnizian optimism.
Fundamental to Voltaire's attack is Candide's tutor Pangloss, a self-proclaimed follower of Leibniz and a teacher of his doctrine. Ridicule of Pangloss's theories thus ridicules Leibniz himself, and Pangloss's reasoning is silly at best. For example, Pangloss's first teachings of the narrative absurdly mix up cause and effect:
Il est démontré, disait-il, que les choses ne peuvent être autrement; car tout étant fait pour une fin, tout est nécessairement pour la meilleure fin. Remarquez bien que les nez ont été faits pour porter des lunettes; aussi avons-nous des lunettes.
It is demonstrable that things cannot be otherwise than as they are; for as all things have been created for some end, they must necessarily be created for the best end. Observe, for instance, the nose is formed for spectacles, therefore we wear spectacles.
Following such flawed reasoning even more doggedly than Candide, Pangloss defends optimism. Whatever their horrendous fortune, Pangloss reiterates "all is for the best" ("Tout est pour le mieux") and proceeds to "justify" the evil event's occurrence. A characteristic example of such theodicy is found in Pangloss's explanation of why it is good that syphilis exists:
c'était une chose indispensable dans le meilleur des mondes, un ingrédient nécessaire; car si Colomb n'avait pas attrapé dans une île de l'Amérique cette maladie qui empoisonne la source de la génération, qui souvent même empêche la génération, et qui est évidemment l'opposé du grand but de la nature, nous n'aurions ni le chocolat ni la cochenille;
it was a thing unavoidable, a necessary ingredient in the best of worlds; for if Columbus had not caught in an island in America this disease, which contaminates the source of generation, and frequently impedes propagation itself, and is evidently opposed to the great end of nature, we should have had neither chocolate nor cochineal.
Candide, the impressionable and incompetent student of Pangloss, often tries to justify evil, fails, invokes his mentor and eventually despairs. It is by these failures that Candide is painfully cured (as Voltaire would see it) of his optimism.
This critique of Voltaire's seems to be directed almost exclusively at Leibnizian optimism. Candide does not ridicule Voltaire's contemporary Alexander Pope, a later optimist of slightly different convictions. Candide does not discuss Pope's optimistic principle that "all is right", but Leibniz's that states, "this is the best of all possible worlds". However subtle the difference between the two, Candide is unambiguous as to which is its subject. Some critics conjecture that Voltaire meant to spare Pope this ridicule out of respect, although Voltaire's Poème may have been written as a more direct response to Pope's theories. This work is similar to Candide in subject matter, but very different from it in style: the Poème embodies a more serious philosophical argument than Candide.
The conclusion of the novel, in which Candide finally dismisses his tutor's optimism, leaves unresolved what philosophy the protagonist is to accept in its stead. This element of Candide has been written about voluminously, perhaps above all others. The conclusion is enigmatic and its analysis is contentious.
Voltaire develops no formal, systematic philosophy for the characters to adopt. The conclusion of the novel may be thought of not as a philosophical alternative to optimism, but as a prescribed practical outlook (though what it prescribes is in dispute). Many critics have concluded that one minor character or another is portrayed as having the right philosophy. For instance, a number believe that Martin is treated sympathetically, and that his character holds Voltaire's ideal philosophy—pessimism. Others disagree, citing Voltaire's negative descriptions of Martin's principles and the conclusion of the work in which Martin plays little part.
Within debates attempting to decipher the conclusion of Candide lies another primary Candide debate. This one concerns the degree to which Voltaire was advocating a pessimistic philosophy, by which Candide and his companions give up hope for a better world. Critics argue that the group's reclusion on the farm signifies Candide and his companions' loss of hope for the rest of the human race. This view is to be compared to a reading that presents Voltaire as advocating a melioristic philosophy and a precept committing the travellers to improving the world through metaphorical gardening. This debate, and others, focuses on the question of whether or not Voltaire was prescribing passive retreat from society, or active industrious contribution to it.
Inside vs. outside interpretations
Separate from the debate about the text's conclusion is the "inside/outside" controversy. This argument centers on the matter of whether or not Voltaire was actually prescribing anything. Roy Wolper, professor emeritus of English, argues in a revolutionary 1969 paper that Candide does not necessarily speak for its author; that the work should be viewed as a narrative independent of Voltaire's history; and that its message is entirely (or mostly) inside it. This point of view, the "inside", specifically rejects attempts to find Voltaire's "voice" in the many characters of Candide and his other works. Indeed, writers have seen Voltaire as speaking through at least Candide, Martin, and the Turk. Wolper argues that Candide should be read with a minimum of speculation as to its meaning in Voltaire's personal life. His article ushered in a new era of Voltaire studies, causing many scholars to look at the novel differently.
Critics such as Lester Crocker, Henry Stavan, and Vivienne Mylne find too many similarities between Candide's point of view and that of Voltaire to accept the "inside" view; they support the "outside" interpretation. They believe that Candide's final decision is the same as Voltaire's, and see a strong connection between the development of the protagonist and his author. Some scholars who support the "outside" view also believe that the isolationist philosophy of the Old Turk closely mirrors that of Voltaire. Others see a strong parallel between Candide's gardening at the conclusion and the gardening of the author. Martine Darmon Meyer argues that the "inside" view fails to see the satirical work in context, and that denying that Candide is primarily a mockery of optimism (a matter of historical context) is a "very basic betrayal of the text".