英：sig; 法：signe
拉康将符号定义为“为某人代表某物”的东西，而能指（SIGNIFER)则与之相对，是“为另一能指代表主体”的东西 (S11,207). 经由着手符号的概念，拉康便把他的著作安置在与符号学 (semiotics)的密切关系之中，后者是在20世纪迅速发展起来的一门学科。在符号学内部可以区辨出两条主要的发展路线：联系于费尔迪南·德·索绪尔的欧洲路线 (索绪尔本人便是以“符号学”的名称将其命名的)，以及联系于查尔斯·桑德斯·皮尔斯的北美路线。
(1)根据索绪尔的观点，符号是语言 (LANGUAGE, langue)的基本单位。符号由两个元素构成：一个概念元素 (索绪尔将其称作“所指”)以及一个语音元素 (被称作“能指”)。这两个元素经由一种任意但牢不可破的联结而被联系起来。索绪尔借由一个图解来表示符号 (图15；见：Saussure, 1916:114)。
在此图解中，箭头代表着意指中所固有的相互蕴涵，而所指与能指之间的线条则代表着结合①。
在1950年代期间，拉康在其精神分析学的“语言学转向”(liguistic turn)中接纳了索绪尔的符号概念，但对其进行了若干修改。首先，索绪尔假设能指与所指之间是相互蕴涵的关系（它们就像一张纸的两面是相互依存的），而拉康则认为能指与所指之间的关系是极其不稳定的 (见：滑动[SLP])。其次，拉康宣称一种“纯粹能指”秩序的存在，这些能指在那里是先于所指而存在的，这一纯粹逻辑结构的秩序即无意识。这相当于是对索绪尔符号概念的一种摧毁，在拉康看来，语言不是由符号而是由能指所组成的。
为了阐明他自己的见解与索绪尔的观点之间的反差，拉康以一则算法 (图16)取代了索绪尔的符号图解。拉康指出，这则算法应当被归于索绪尔（因而它现在有时候被称为“索绪尔式算法”一见：E, 149)。
在图16中，S代表能指，而s则代表所指：所指与能指的位置因而颠倒了过来，从而表明了能指的优先性 (能指以大写字母来书写，而所指则被降格为仅仅是小写字母的斜体字)。箭头与圆圈遭到废除，从而表明在能指与所指之间缺乏一种稳定或固定的关系。能指与所指之间的那道杠 (BAR)不再表示结合，而是表示意指中固有的阻抗。在拉康看来，这则算法定义了“无意识的地形学”(E, 163).
(2)根据皮尔斯的观点，对某一解释者来说，符号是代表一个对象的东西 (对皮尔斯而言，“对象”一词可能意味着一个物体、一则事件、一种观念或是另一符号)。皮尔斯把符号划分为三种类型：“象征符”(symbols)、“指示符”(indices)以及“肖似符”(icos), 它们皆以不同的方式联系着对象。象征符与它所指涉的对象虽然没有任何“自然”的或必然的关系，却经由一种纯粹约定俗成的规则而联系于对象。指示符 (NDEX)与它所代表的对象具有一种“存在判断关系”（即指示符总是在空间上或在时间上临近于对象）。肖似符则通过经由相似性展示其形式来表现一个对象。皮尔斯在肖似符、指示符与象征符之间做出的这些区分皆是分析性的，而非意在相互排斥。因此，一个符号便几乎总是会以各种模式而运作，例如，人称代词便既是象征性又是指示性运作的符号 (见：Peirce, 1932:156-73: Burks, 1949).
拉康接纳了皮尔斯的指示符概念，以便在精神分析与医学的症状概念之间做出区分，并且在（动物的）编码与（人类的）语言之间做出区分。拉康同样还沿着罗曼·雅各布森布下的线索，用转换词 (SHIFTER)的概念发展了指示符的概念，从而在所述的主体 (subject of the statement)与能述的主体 (subject of the enunciation)之间做出了区分。
(signe) Lacan defines the sign as that which 'represents something for someone', inopposition to the SIGNIFIER, which is 'that which represents a subject for anothersignifier' (S11,207). By engaging with the concept of the sign, Lacan sets his work inclose relation to the science of semiotics, which has grown rapidly in the twentiethcentury. Two main lines of development can be discemned within semiotics: the Europeanline associated with Ferdinand de Saussure (which Saussure himself baptised with thename of 'semiology), and the North American line associated with Charles S.Peirce.
- According to Saussure, the sign is the basic unit of LANGUAGE (langue). The signis constituted by two elements: a conceptual element (which Saussure calls the signified), and a phonological element (called the signifier). The two elementsare linked by an arbitrary but unbreakable bond. Saussure represented the sign by meansof a diagram (Figure 17; see Saussure, 1916:114).
In this diagram, the arrows represent the reciprocal implication inherent insignification, and the line between the signified and the signifier represents union.
Lacan takes up the Saussurean concept of the sign in his 'linguistic turn'inpsychoanalysis during the 1950s, but subjects it to several modifications. Firstly, whereas Saussure posited the reciprocal implication between the signifier and the signified (theyare as mutually interdependent as two sides of a sheet of paper), Lacan argues that therelation between signifier and signified is extremely unstable (see SLIP). Secondly, Lacan asserts the existence of an order of 'pure signifiers', where signifiers exist prior tosignifieds; this order of purely logical structure is the unconscious. This amounts to adestruction of Saussure's concept of the sign; for Lacan, a language is not composed ofsigns but of signifiers.
To illustrate the contrast between his own views and those of Saussure, Lacanreplaces Saussure's diagram of the sign with an algorithm (Figure 18) which Lacan argues, should be attributed to Saussure (and is thus now sometimes referred to asthe 'Saussurean algorithm'-see E, 149).
The S in Figure 18 stands for the signifier, and the s for the signified; the position ofthe signified and the signifier is thus inverted, showing the primacy of the signifier (which is capitalised, whereas the signifier is reduced to mere lower-case italic). Thearrows and the circle are abolished, representing the absence of a stable or fixed relationbetween ssignifier and signified. The BAR between the signifier and the signified nolonger represents union but the resistance inherent in signification. For Lacan, thisalgorithm defines 'the topography of the unconscious' (E, 163). 2. According to Peirce, the sign is something which represents an object tosomeinterpretant (the term 'object'can mean, for Peirce, a physical thing, an event, an idea, oranother sign). Peirce divides signs into three classes: 'symbols','indices'and 'icons', which differ in the way they relate to the object. The symbol has no 'natural'or necessaryrelationship to the object it refers to, but is related to the object by a purely conventionalrule. The INDEX has an 'existential relation'to the object it represents (i.e.the index isalways spatially or temporally contiguous to the object). The icon represents an object byexhibiting its form via similarity. Peirce's distinctions between icons, indicesandsymbols are analytical and not intended to be mutually exclusive. Hence a sign willalmost always function in a variety of modes; personal pronouns, for example, are signswhich function both symbolically and indexically (see Peirce, 1932:156-73; Burks, 1949).
Lacan takes up Peirce's concept of the index in order to distinguish between thepsychoanalytic and medical concepts of the symptom, and to distinguish between (animal) codes and (human) languages. Lacan also develops the concept of the indexalong the lines set down by Roman Jakobson in the concept of the SHIFTER, todistinguish between the subject of the statement and the subject of the enunciation.