How do you know that you have a body?
Do you see it with your body's eyes?
Do you hear it with your body's ears?
Do you taste it with the body's tongue, touch it with the body's flesh, and smell it with the body's nose?
Is this not a completely circular logic within a closed system?
You are using the very device that you are trying to detect, to detect that same device.
It cannot see outside of its own self. It cannot objectively report whether its own form exists or not.
Its own eyes show you a picture of a body, but the picture is seen with the body that the body is looking at. And so using the thing itself to detect itself, is completely invalid.
You would need proof outside of your own body, that your body exists. And you have none. A scientist could come along and tell you that, per their study, your body does in fact exist. But this would also be a lie. Because they used bodies to look at bodies. They uses eyes to look at eyes. It's entirely invalid within a closed loop.
The body therefore cannot be relied upon to tell you of its own existence. And therefore, there is no proof whatsoever that the body really exists.
You think you see your body in a mirror. But your body is using itself to look at itself. It is completely biased. It cannot tell you if it exists or not, it is designed ONLY to report a false positive that it "has been sensed". It's not capable of telling you of its own non-existence, and is totally one-sided.
The body does not really exist. All it can do is sense what it was designed to sense and NOT see reality.
"To sense is not to know." - ACIM
"The body is not the means by which the real world can be seen." - ACIM
"The body is the instrument the mind made in its striving to deceive itself." - ACIM
"At no time does the body really exist." - ACIM
"Only the body makes the world seem real, for being separate it could not remain where separation is impossible. Forgiveness proves it is impossible because it sees it not." - ACIM
"Here is a world established that is sick, and this the world the body's eyes perceive. Here are the sounds it hears; the voices which its ears were made to hear. Yet sights and sounds the body can perceive are meaningless. It cannot see nor hear. It does not know what seeing IS; what listening is FOR. It is as little able to perceive as it can judge, or understand, or know. Its eyes are blind, its ears are deaf. It can NOT think, and so it cannot HAVE effects." - ACIM
"The body is clearly a device for separation and therefore does not exist." - ACIM
"The ultimate purpose of the body is to render itself unnecessary. Learning to do this is the only real reason for its creation." - ACIM
"Minds need not bodies to communicate." - ACIM
"To be without a body is to be in your natural state." - ACIM