Many of the ideas in the following text were conceived by Roberto Casati, some of them are derived from the books found in the links and some are my own.

In the Brockhaus Encyclopaedia shadow is defined as “the space with no or little light behind a reflecting, light absorbing or opaque body”. This means that shadow is primarily a space or a volume which we usually call a shadow only if it is clearly circumscribed. The English language differentiates between those two aspects by using “shadow” to denote a two-dimensional shape or projection, and “shade” to indicate a three-dimensional space. Other languages use one and the same word but express the difference through the context. Thus, in Italian “nell ombra” is used to indicate a defined shadow and “all ombra”, to denote the shade.

Shade and shadows may be further differentiated. Thus, there is the full shade where no light waves are present, and the half shade or twilight where only refracted light reaches. Also, there are those parts of an object that are not exposed to the light but lying on the side of the shadow that is thrown by the object itself. In a global context, the full shade on that part of the earth that is averted from the light coming from the sun is called night.

Shadows enable us to identify spatial dimensions and structures and help us to analyse simple objects. They provide information on the nature of objects and on the positions of light source, objects and plane structures. Tiny shadows even allow us to discern the surface texture of objects.
Without shadows the world would be flat and dull, and much of its three-dimensional structure would be indiscernible. Yet in our perception we use shadows only in a rudimentary and superficial manner and quickly dismiss them from our field of vision. Their complexity does not facilitate the rapid absorption of our environment that is necessary for fast orientation and reaction. It is impossible to perceive all the shadows surrounding us without pausing for a moment. To consciously see and study the world of shadows requires a whole lot of concentration.

Shadows are evanescent and intangible. They are no material phenomenon, but they are present in all dimensions. Shadows come and go without leaving a trace. In the moment of its existence the shadow is the proof of an encounter with light. Its fleeting existence depends on a minimum of light.
Shadows are without any memory and do not tell a story. The past does not leave its imprint on their intangible nature. Shadows represent the present moment in its purest form and are subject to the flow of time. Therefore they are different at every moment and at the same time subordinate to the object whose representations they are. The shadow is a slave that has to follow at every turn, but is not always in attendance. It disappears and reappears again, it changes its size, it clings tenaciously to its body but it can never be captured.

The shadow is pure form without matter, an insubstantial reduction of the object it is caused by. It has a volume, but it is flat and colourless when circumscribed. Its outline encompasses an undefined core. And above all it is – according to the first theory (cf. page 2) – the absence of light.
But this absence of light makes it a hiding place as well, because our eyes cannot penetrate into the dark. Our eyes can only see what the source of light highlights, and the shadow hides where the light does not reach.

It is the nature of the shadow to vanish when it is exposed to the light. Only the sun has an unclouded view of the world and has never caught sight of a shadow. According to the first theory, shadows are conditional phenomena. There are no shadows without the objects that throw them. The object that throws a shadow is active, the shadow is passive. If the object is moved, the shadow moves, not the other way round. The shadow is controlled by movement and the angle of the incident light.

Shadows are parasites that consist of nothing. Intangible parasites, immaterial entities that irritate our mind.
It may be helpful to describe the shadow as the absence of light, of something that is in itself ephemeral and hard to comprehend. But what is more: the shadow is a localised absence of light whose boundary has to be perceivable. For our mind to realise the shadow’s existence there has to be a boundary that clearly separates light and shade. Night is not registered as shade in our understanding of the world. This fact was a discovery that had to be explicitly made. Large phenomena in the sky or in the cosmos are associated with darkness rather than with shadow since their dimensions exceed our capacity of perception.

According to the first theory shadows may be likened to holes, because holes are also subordinate phenomena. There is no hole without an object that contains the hole. The object is the host, as it were, and the hole is the parasite. Holes also consist of nothing. And a hole also is a local absence that needs a boundary, a border to the host object, beyond which the hole opens up. But holes are parasites of static hosts, while shadows are holes in the light which is dynamic by nature. Thus, shadows are flying holes which connect the object that throws the shadow with the projected surface area of the shadow.

Shadows are images, reflections of the object that throws the shadow even though we may not be able to recognise the object from its shadow. A shadow is the trace that an object leaves in the light that falls on it.
The shadow leads a double life, as a three-dimensional body and a two-dimensional figure. Sometimes we may think it extends from the object to the projected area, then again we reduce it to the area it is thrown on.
And finally, the nature of the shadow coincides with the nature of light in astonishing and unsettling ways. In some situations there is no appropriate way to differentiate between them. If you place green glass between a source of light and a projection screen, it is not clear whether the resulting green blotch is a blotch of light or a blotch of shadow. There is no conclusive answer to this question.
Shadows have always had a special position. We tend to overlook them and to mention them only in passing. They play their part, but they do not receive any attention. The shadow is like a secret guest that whizzes by and remains completely unnoticed.

I would like to end by quoting Roberto Casati:

I have carefully watched out for shadows and I have noticed sea gulls flying backwards, and palm trees gently swaying in the wind even after they had been cut down. I have seen balconies stretching out endlessly, and houses swallowing them up whole, I have seen giant men shrink to little children and children turn into giants. I have seen trees lunging into the water and crossing rivers, I have seen eaves gutters and down pipes turn into veils covering whole walls. Agile like a blackbird I have climbed up to the highest top of cypress pines swaying in the wind. I have seen women moving through each other, I have followed the course of the sun in a bowl. I have learned that light can turn into shadow and shadow into light.

These were fleeting encounters, cancelled out by the course of the sun, the sequence of day and night. Sometimes I feel like I am the only one on earth who has the privilege to witness such extraordinary and incredible events. I am aware that these events occur in front of everyone but remain unnoticed. Sometimes I seem to be the only one in this parallel universe. After long years of apprenticeship I have learned to find my way around this universe, I have surveyed it topografically, like someone investigating a new world, a world that is overlapping with ours but remains untouched, a world I can enter and leave at every corner, indeed at every step without ever weaving a spell.