"Medieval philosophers were fascinated by mirrors. They inquired in particular into the nature of the images that appear in them: What is the being, or rather the non-being, of these images?
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Two characteristics are derived from the insubstantial nature of the image. Since the image is not a substance, it does not possess any continuous reality and cannot be described as moving by means of any local movement. Rather, it is generated at every moment according to the movement or the presence of the one who contemplates it: "Just as light is always created anew according to the presence of the illuminator, so do we say that the image in the mirror is generated each time according to the presence of the one who looks." The being of the image is a continuous generation (semper nova generatur), a being [essere] of generation and not of substance. Each moment, it is created anew, like the angels who, according to the Talmud, sing the praises of God and immediately sink into nothingness.
The second characteristic of the image is that it cannot be determined according to the category of quantity; it is not, properly speaking, a form or an image but rather the "aspect of an image or of a form" (species imaginis et formae). In itself, it cannot be described as long or wide, but instead as "having only the aspect of length and width." The dimensions of the image are therefore not measurable quantities but merely aspects or species, modes of being and "habits" (habitus vel dispositiones). This characteristic - being able to refer only to a "habit" or an ethos - is the most interesting signification of the expression "being in a subject." What is in a subject has the form of a species, a usage, a gesture. It is never a thing, but always and only a "kind of thing" [specie di cosa].
The Latin term species, which means "appearance," "aspect," or "vision," derives from a root signifying "to look, to see:' This root is also found in speculum (mirror), spectrum (image, ghost), perspicuus (transparent, clearly seen), speciosus (beautiful, giving itself to be seen), specimen (example, sign), and spectaculum (spectacle). In philosophical terminology, species was used to translate the Greek eidos (as genus was used to translate genos); hence the sense the term takes on in natural science (animal or plant species) and in the language of commerce, where the term signifies "commodities" (particularly in the sense of drugs and spices) and, later, money (especes).
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The mirror is the place where we discover that we have an image and, at the same time, that this image can be separate from us, that our species or imago does not belong to us. Between the perception of the image and the recognition of oneself in it, there is a gap, which the medieval poets called love. In this sense, Narcissus's mirror is the source of love, the fierce and shocking realization that the image is and is not our image. If the gap is eliminated, if one recognizes oneself in the image but without also being misrecognized and loved in it - if only for an instant - it means no longer being able to love; it means believing that we are the masters of our own species and that we coincide with it. If the interval between perception and recognition is indefinitely prolonged, the image becomes internalized as a fantasy and love falls into psychology.
In the Middle Ages, species was also called intentio, intention. The term names the internal tension (intus tensio) of each being, that which pushes it to become an image, to communicate itself. The species is nothing other than the tension, the love with which each being desires itself, desires to persevere in its own being. In the image, being and desire, existence and conatus* coincide perfectly. To love another being means to desire its species, that is, to desire the desire with which it desires to persevere in its being.
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The transformation of the species into a principle of identity and classification is the original sin of our culture, its most implacable apparatus [disposiuvo]. Something is personalized - is referred to as an identity at the cost of sacrificing its specialness. A being - a face, a gesture, an event - is special when, without resembling any other, it resembles all the others.
Giorgio Agamben Profanations (2007) p.55-58
* striving - an innate inclination of a thing to continue to exist and enhance itself