Form and substance are closely related in Aristotelian–Thomistic metaphysics:
Substance = what something is in itself (its essence as an individual being).
Form = the intrinsic principle that makes a thing the kind of thing it is (its essence as a nature).
You can think of form as the blueprint, and substance as the completed thing built from that blueprint and matter.
So, in one sense, form gives rise to substance: the union of form + matter is the substance. That’s why they can sometimes be treated together as shorthand.
All say that it is not natural for the form of a thing or body to be separated from the thing or body. Consider the Catholic doctrine of the Mass. In the Mass, the bread and wine without physically or discernably changing supposedly become the body and blood of Jesus. In the Mass, the bread and wine cease to be what they were — their form and substance as bread and wine are replaced by the real presence of Christ Himself.”The form of bread and wine is changed into the form of Jesus. In other words, what makes bread bread and wine wine is changed into what makes Jesus Jesus. So they reason that God's intervention is at work. To be technical, God changes the substance of bread and wine into Jesus without changing the accidents. Think on this. It is not natural for a brick to just float in the air. But if it were more common, we would soon start calling it natural while admitting we do not know. Frequency doesn’t redefine what’s natural; naturalness is about what follows from created causes, not what happens “commonly. That is true. But the problem is that if we cannot detect the causes then we will treat the frequent as an indicator of what is natural.
In Catholic metaphysics, it’s considered coherent for God to act miraculously in some cases while nature proceeds normally in others. That argues that it’s not an arbitrary assumption but a distinction between two orders of causality - natural and supernatural. This does not work. Exceptions have to prove the rule and you never know if a priest really let God change bread and wine at any Mass. The teaching is that the priest does not intend for God to work the miracle of transubstantiation then nothing happens.
So how do Catholics know that the disconnect between thing and form is not happening regularly in most things? Take the human person supposedly, a body and soul, the form. How do they know that the soul and the body are not already separate and God is not miraculously uniting them? They assume the form and body go together but it is a mere assumption. If a miracle might explain their separation there is no reason to think that separation might be natural. Their assuming is nothing and they cannot oblige us to agree with them.
Even granting that Jesus meant His body and blood are truly received, the change (if any) should logically occur at the moment of consumption, not beforehand on the altar. That makes sure that no unnecessary changing is done.
This interpretation aligns with:
John 6:53–56, where the focus is on eating and drinking Christ, not on the nature of the elements themselves.
1 Corinthians 10:16, which speaks of “the cup of blessing which we bless” as “a participation in the blood of Christ” — again emphasizing participation, not a metaphysical change in the elements.