When we deny Him and deny we are His Son, we stop extending Him (as source of everything) and instead have to 'make up' stuff which we think WE are the source of. This is how we 'make' the world and the body.
We see ourselves as the starting point of creation because we do not extend God and do not receive from God, and so we think reality originates with us. Then we make stuff.
The problem is, the stuff we make, takes after ourselves. Like 'Father' like 'Son', we make things which CANNOT extend us. Since we believe our nature is not to extend what created us (God), we must believe that the things we MAKE without God also cannot extend.
So we make things which, instead of having properties of being able to, in turn, have creative power, we make things which become 'dead ends'. Things which have no creative potential. Things which cannot extend life. Things which are stuck being only what they seem to be, in form, and which have no (or very limited) life.
The physical world of form, which we made, is an exact example of this. It is a world where we make 'creations' which barely have any semblance of their maker, and which have very little creative power, and which therefore constantly limit what they create.
We as their maker - the maker of the world - put so much limitation on ourselves in order to shut out God, that we create `limited things`. These things have very little freedom or life and so take on the appearance we are familiar with - inanimate objects and creatures that only live a short time then die.
As the Course says, anything made without the Father's Life cannot be sustained and withers and dies. Such is the physical world that WE made, not God.
Now, once we've 'made' stuff which barely extends life, we recognize it has no creative power. It's just a dream. BUT, since we are on a higher level (mind) than form, WE still believe in extension more than our made-up creations do. We still believe in enough of an idea of giving our creations the power to CAUSE, that we retain a certain belief that whatever we've made can 'do something'.
Unfortunately, since there is great limitation to what we made, and it can't really do much at all, when we try to project this idea of causal power onto it, we imagine that it has 'magical powers'. Powers to create that it does not have. And this directly creates LEVEL CONFUSION, which is MAGIC, and which is a reversal of cause and effect, because now this weak, powerless, lump of matter seems to us to have (or is supposed to have) causal power to further creation.
Believing that something that has no power, has power, is magic. And believing it can cause, when essentially it is what we call an 'effect' (effects that have no cause in them - separation of cause and effect), we have to believe that these forms and objects have a mind of their own, a will to create, a causal power over us, and now we are at their effect.
This produces all sickness.
Creating without the Father, without extending His love, produces sickness. And that sickness takes on form because we're 'making magical illusions'.
Something like that, anyway