Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

Since both the term and discourse and proposition and the other logical instruments are defined through signification, because the intellect knows by means of significant concepts and speaks by means of significant words, and, universally, all the instruments we use for knowing and speaking are signs, therefore, in order that the logician know his instruments exactly—that is, terms and discourses—he must also know what a sign is. A sign, then, is defined in general as “that which represents something other than itself to a cognitive power.”¹

For this definition to become clearer, one must consider that there are four causes of cognition: efficient, objective, formal, and instrumental. The efficient cause is the power itself that elicits cognition, such as the eye, hearing, or the intellect. The object is the thing that moves cognition or toward which cognition tends, as when I see a stone or a human being. The formal cause is the very cognition by which the power is made cognitive, such as the very vision of a stone or a human being. The instrumental cause is the means by which an object is represented to a power, as an external image of Caesar represents Caesar. An object is of three kinds: merely moving, merely terminating, or both moving and terminating. That which merely moves is what moves a power to form a cognition not of itself but of another, as an image of the emperor moves one to know the emperor. That which merely terminates is the thing known through a cognition produced by another object, as the emperor known through the image. That which both terminates and moves is what moves a power to form a cognition of itself, as when a wall is seen in itself.

Therefore, “to make known” has a wider scope than “to represent,” and “to represent” a wider scope than “to signify.” For to make known is said of everything that contributes to cognition, and is thus said in four ways: efficiently, objectively, formally, and instrumentally. Efficiently, it is said of the power itself that efficiently causes cognition and of the causes that contribute to it, such as God moving, the agent intellect generating species, an assisting habit, and so forth. Objectively, it is said of the thing itself that is known. For example, if I know a human being, the human being, as object, makes himself known by presenting himself to the power. Formally, it is said of the cognition itself, which makes the knower cognitive as a form. Instrumentally, it is said of the very means that conveys the object to the power, as an image of the emperor conveys the emperor to the intellect as a kind of means; this means we call an instrument. To represent is said of everything by which something is made present to a power, and is thus said in three ways: objectively, formally, and instrumentally. For an object represents itself objectively, as a wall does; cognition represents formally; a trace² represents instrumentally. To signify is said of that by which something distinct from itself is made present, and is thus said in only two ways: formally and instrumentally.

Hence arises a twofold division of sign. Insofar as a sign is ordered to a power, it is divided into formal and instrumental sign; insofar as it is ordered to what is signified, it is divided according to the cause that orders it into natural, conventional, and customary sign. A formal sign is a formal cognition that represents by itself, without the mediation of another. An instrumental sign is that which, from a pre-existing cognition of itself, represents something other than itself, as the trace of an ox represents an ox. This definition is commonly given for sign³. A natural sign is that which represents by the nature of the thing, with every imposition and custom set aside; and thus it represents the same thing to everyone, as smoke does fire. A conventional sign is that which represents something by an imposition of the will through public authority, as the word “human being.” A customary sign is that which represents by use alone, without public imposition, as tablecloths on a table signify a meal. We treat all these matters concerning the nature and division of signs at length in Logic, questions 21 and 22.