When at his Last Supper Jesus said: "This is my body", what he held in his hands had all the appearances of bread. However, the Catholic Church teaches that the underlying reality was changed in accordance with what Jesus said, that the "substance" of the bread was converted to that of his body. In other words, it actually was his body, while all the appearances open to the senses or to scientific investigation were still those of bread, exactly as before. Science has no direct dealing with substance, but only with appearances – and in these, by the very terms of the dogma, there is no change.
