Substance and accidents confuse me

In Transubstantiation we have substance and accidents. Would form and matter be subcategories under substance?
I just read that we are talking about a substantial conversions and not a formal change. What would be the difference between those two conversions and hiw is conversion different from change?

Aquinas adopted Aristotle’s terminology of “substance” and “accidents” as part of his larger project for ironing out the apparently irreconcilable differences between Aristotelian philosophy and the Catholic faith. Nowadays we no longer use these terms, as Aristotle did, to explain the phenomena of the material world.

If we are trying to explain to a nonscientist why we see the flash of lightning before we hear the thunder, or how a cow turns grass into milk, or why a can goes rusty if we leave it out in the rain, we no longer explain those things in terms of “substance” and “accidents”. But Aristotle did, which is why Aquinas decided he needed to explain the Eucharist, too, using the same Aristotelian terminology.

When we try and use the terms “substance” and “accidents” to explain the Catholic doctrine of the Eucharist to a nonbeliever, we run into the difficulty that we first have to explain what the two words themselves mean. The trouble is that in our present-day world these terms are now used only in connection with the Eucharist.


Conversion of substance is called transubstantiation. In transubstantiation the accidents are sustained by divine power, even though the substance has changed.

Accidents: the appearances, species, or properties of a thing.
Substance: A being whose essence requires that it exist in itself.

Matter: That which constitutes a body. (That which can be measured in space and time.) This is the principle of individuation.
Form: The nature or essence of a thing. This is the principle of intrinsic nature.

Ref: Modern Catholic Dictionary


Then what is “formal conversion”?

And what is the difference between form and substance?

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