Does the presence of a shadow preclude the existence of the object?

Yes. A shadow is something. It is a signature of light absorption by the object. One does not require mass to be something.

Is a thought unreal? How about speech?

A vacuum is the absence of all matter, but a vacuum in quantifiable.

The idea that shadows do not exist is erroneous. The word shadow refers to something. I saw a shadow the other day, I swear to it! Since when did absence of physical light mean nonexistence? Geez! Besides, if you can see a shadow then there is some light!

The mere fact that you noticed the shadow attested to the presence of an object. You see, it was not your eyesight (one of the 5 senses) we are questioning, it was the other one of your senses that was working in this moment, the 6th sense.

Intuition is part of our survival mechanism. Immediately think- "what is it, what am I being warned about? Is there danger?

Pay attention to it, because there are many shades of darkness and many shades of light. Trying to explain the matter with science is kind of - well ----lame.

The word 'unreal' is only applicable to psychological illusions, and even THEY cause actual physical manifestations in the form of experience.

You need to start over with another analogy or metaphor if you want to discuss the nature of reality.

What if the shadow is following the object?

Wow, I agree with the first poster. I wish I had thought to say it as eloquently as he did.