Trent

as to ...'When St Thomas wrote of the Real Presence of Christ in the Eucharist "...

I wonder how many people understand the Aristotelian Philosophy that St  Thomas Acquinas used?

Not sure that I do; but as far as I can figure, that Latin 'substantia' has not quite the same meaning as our English word 'substance' or 'substantial'.

'Sustantia', in that philosophy, is a mental category. Eg. –Something you sit on is a 'chair'. But if you take 4 chairs , attach them together, and remove the center arms so that 4 or 5 people sit on it, then you have 'trans-substantiated' those 'chairs' into a 'bench'. You have a different 'mental category'.

That's what  St.Thomas means by 'tran-substantiation'. The bread and wine of the Eucharist become 'consecrated' bread and wine and so a communion with, and remembrance of,  the sacrificed, risen, glorified Jesus – body and  blood, soul and Divinity. The mental category has changed.

Our English 'substance' carries a different connotation.