The caller to Catholic Answers Live identified himself as an Evangelical, and he had a question about the Eucharist. He wanted to know whether anyone had ever performed a DNA test on a host to prove (or disprove) the presence of human flesh.
Not so far as I knew, I said. Besides, such a test would be a waste of time because it would turn out negative. No need to rouse the scientists from their slumbers.
Before the caller could say "gotcha," I turned to philosophy. I noted that the jeans I was wearing that day in the radio studio were blue, but their blueness was incidental to their essence. Aquinas and Aristotle would have said that blueness was one of the accidents of the jeans. Nowadays we might prefer to say appearances because accidents brings up the unhelpful image of cars colliding.
A thing’s appearances are the properties that appear to the five senses: sight, touch, smell, hearing, taste. My jeans are blue, soft, sweet-smelling (because freshly laundered), silent, and—here I guess—bland. But these properties are not the essence of the jeans. The jeans’ essence is their "jeansness," what they are in their inner nature, apart from their changeable appearances.
As another example, I cited a tree. A tree is tall, brown, rough, fragrant, and so forth. Those are the properties accessible to our senses, but the inner nature or essence of a tree, according to this old way of philosophizing, is its "treeness." If we alter the appearances, we simultaneously alter the essence of the tree, and it becomes something else. If we burn it down and reduce it to ash, it no longer is tall, brown, rough, and fragrant, and it ceases to have the essence of a tree and now has the essence of ash instead.
In our everyday experience, a change in appearances always is accompanied by a change in essence, and a change in essence always is accompanied by a change in appearances. But, at the strictly philosophical level, there is no reason the one must be found with the other. It is possible to conceive of something that maintains its appearances while its essence changes. While this would seem to be contrary to the laws of nature, it is not self-contradictory and therefore is something God could arrange, if he wished to.
And he did wish to, in the case of the Real Presence of Christ in the Eucharist. We know this because Christ told us so, in the sixth chapter of John’s Gospel.
I told the Evangelical caller that the Catholic Church takes literally what its founder clearly meant to be taken literally in that Gospel: Ordinary bread would be transformed into his own body. We know this change occurs, because Christ—who can neither deceive nor be deceived—has told us so. But our eyes verify that the appearances of the bread do not change at all. Put a consecrated host under a microscope, and you find what appears to be plain bread. You will see no human cells under the lens, and a DNA test will show—nothing.
So we have a change without a change: The essence is changed (the bread is now Christ’s body) but the appearances are not (his body still looks like bread). This is a mystery. The word does not mean that it is inexplicable (after all, I just explained it to you!) but that, while accepting the explanation, we admit that we cannot understand how such a change can happen. All we can do is affirm, with pious trust in Christ, that it does happen and then act accordingly.