The Commens Dictionary
Quote from ‘Harvard Lectures on Pragmatism: Lecture III’
The Genuine Index represents the duality between the Representamen and its Object. As a whole it stands for the Object; but it has a part or element of it [which it] represents as being the Representamen, by being an Icon or analogue of the Object in some way; and by virtue of that duality, it conveys information about the Object. The simplest example of a genuine Index would be, say, a telescopic image of a double star. This is not an Icon simply, because an Icon is a representamen which represents its object solely [by] virtue of its similarity to it, as a drawing of a triangle represents a mathematical triangle. But the mere appearance of the telescopic image of a double star does not proclaim itself to be similar to the star itself. It is because we have set the circles of the equatorial so that the field must by physical compulsion contain the image of that star that it represents that star and by that means we know that the image must be an icon of the star, and information is conveyed. Such is the genuine or informational index.
In PPM: "...solely be virtue of its similarity..."