The word transubstantiation requires an understanding of the theological and philosophical meaning of the 2 terms - substance and accidents - as they apply to something that exists.
**1) Substance **is the thing itself; it exists in itself, not in something else. Substance is the whole thing; not just a part of it.
2) Accidents cannot exist on their own; they are contingent - can only exist in something/in a substance. (“looks, taste, feel” are accidents. Eg. “red” cannot exist just on it’s own-a jar of red, or an ounce of red, etc. Same with “sweet” (taste) or “soft” (feel), or any of the other accidents. Accidents inhere in substances and so normally when we “know” the accidents of something, we are knowing the substance also.
Here, in my words, is what happens at the words of Consecraion - according to the concepts Aquinas gives in his Summa (to keep it simpler and shorter I’ll just include the bread and not the wine):
- First (prior to Consecration) there is bread –
the **“substance” **is bread;
the “accidents”/species of that substance include the color white, the shape of a circle; the dimensions of that circle; it’s weight, taste,…
- When the priest says the words of consecration over the bread, God takes it’s substance and converts it into the substance of Jesus’ body - but the conversion of the bread terminates right at that point and does not continue on to converting the accidents of the bread.
By God’s power and will, the accidents of bread miraculously remain without inhereing in any substance. These accidents (taste,shape,etc.) really continue to exist and so when you receive the host/bread you experience them.
- But Jesus is present whole and entire in the consecrated host which means therefore that the accidents of His body must also be there since they inhere in it’s substance. They are obviously not present in the manner of physical species. Aquinas terms the manner in which they are present as “concommitance”. (Concommitance is also the manner in which Our Lord’s soul, divinity, and blood are also present in the consecrated bread.) Nevertheless, since the substance of Christ’s body is not really deprived of its dimensive quantity and its other accidents, hence it comes that by reason of real concomitance the whole dimensive quantity of Christ’s body and all its other accidents are in this sacrament.
newadvent.org/summa/4076.htm#article1
I hope this helps. It got longer than I wanted it to.
Just a couple points in closing:
First, it’s important to remember that the Eucharist is a mystery - something we cannot fully comprehend - and it is a miraculous phenomena. We know and believe it because Jesus told us it is Him.
Second, I do not have a degree in philosophy or theology (just done a lot of reading on my own), and there are a lot of gaps because of trying to condense some difficult concepts into just a few lines.