promethius has ably responded to this inquiry (thanks, dude), but I would like to add an additional perspective.
Suppose that I proposed an alternative mechanism by which Transubstantiation might occur. I propose that Angels are equipped with transporter devices (similar to Star Trek), and have access to an unlimited supply of the Body and Blood of Jesus (the angels are, of course, in heaven, where Jesus is present to an infinite degree). They have radio scanners tuned to every Church, and at the words of Consecration they “beam up” the simple bread and wine right off of the altar, and “beam down” the Body and Blood of Jesus. Their transporters are instanteneous (or maybe the angels also employ a Buck Rogers-style “mind ray” that prevents us from noticing any difference).
My proposal is deliberately silly, but it cannot be scientifically proven or disproved (science has no knowledge of the abilities or limitations of angels). My idea does not contradict the doctrine of Transubstantiation in any way (and, in fact, supports it more firmly than the ideas proposed by St. Thomas - we now have angels with transporters!)
But, though my silly idea does not contradict Transubstantiation, it could not possibly be confused with Church doctrine. St. Thomas also had an idea, which was NOT silly (and, which I personally favor), but neither of our ideas have been endorsed by the Catholic Church. Any Catholic is as free to reject my idea of heavenly transporters as they are free to reject St. Thomas’ philosophy.
The thing that no Catholic is free to reject is Our Lord’s assurance that “This is my Body,” and “This is my Blood.” This is what the Catholic Church teaches, and this is all that a faithful Catholic is expected to accept.