"At no single instant does the body exist at all. It is always remembered or anticipated, but never experienced just now. Only its past and future make it seem real. Time controls it entirely, for sin is never present." - ACIM
Although it can seem like you can look at your body "right now" and see it "now", you might not realize that this is not actually "now" that you are seeing it. Rather, you are seeing how it was just a moment ago.
Even in terms of the body's sensory organs itself, it takes "time" for signals of sight to travel through space into the eyes, through the optic nerve, into the brain, into the mind, to perceive what is out there. You do not actually see the body as it is immediately now, you see how it just was a moment ago. The body is always in the past for you.
It is never possible to experience or see the body as it would be immediately in the exact present. Presence requires absolutely no space and no time. It is immediate and direct, with no gap and no delay. In such a present, it is not even possible to perceive. Perception happens in time and space and relies on separation. Only awareness can be present - an awareness of the presence of God.
While you can do a good job of approaching the present, you'll find that you have a lot of trouble actually being squarely in it. For example you can watch your breathing and try to always keep up with exactly how the breathing is acting right now, in the present. But you'll find that instead of always flowing along in the present moment precisely, your awareness will lag behind. You'll attempt to attach to or grab hold of what you're perceiving, and will stutter along from one grasp to another, as the mind tries to get a grip of what's coming. It's anticipating the future, or it's holding onto the past, and slightly missing the immediate present.
To put it another way, the present happens slightly sooner than your ability to perceive it. It is always quicker than the ego. It is always more immediate and transcends time and space. You will not be able to latch onto it or keep up with it without complete surrender to the Holy Instant.
The body does not exist in the present. It exists as a manifestation of delay and avoidance. It is always slightly in the past, or being anticipated in the future. Even though there is what seems to be an illusion of it existing "now" it is not really now. You never see it now. This is also why Jesus says that it is impossible to see a sinless body, because sinlessness is so immediate that it transcends time and space and the body itself.
The body IS the idea of sin, and it is always separate from the presence, because sin is always separate from the presence of God. The body will always be sin and made of sin. It cannot be truly holy. It can be used for a holier purpose but of itself, it is not made by love and cannot be inherently holy, because it is not wholeness. This is why, being sin, it MUST disappear, in order to enter the eternal presence of God. It is an illusion which must be given up and let go. Otherwise if the body were capable of perfection, it would truly substitute for God and we would never wake up.
The ego is very clever at creating illusions. It very cleverly makes it seem as though there IS no present. There's an entire section in the course about how the ego uses the past to "gloss over" the present and extend the past into the future so that there appears to be no present. Even if you are looking at this text right "now" and seeing it "now" it is NOT happening now. You're experiencing the past. This is why the workbook says "I see only the past" or "I see nothing as it is now". It's not possible TO see anything as it is now, other than God. The entire world is therefore in the past and so is already over.
We are never present in this world. This world and presence are incompatible.