" In a closer logical analysis of Transubstantiation, we find the first and fundamental notion to be that of conversion, which may be defined as “the transition of one thing into another in some aspect of being”. As is immediately evident, conversion (conversio) is something more than mere change (mutatio). Whereas in mere changes one of the two extremes may be expressed negatively, as, e.g., in the change of day and night, conversion requires two positive extremes, which are related to each other as thing to thing, and must have, besides, such an intimate connection with each other, that the last extreme (terminus ad quem) begins to be only as the first (terminus a quo) ceases to be, as, e.g., in the conversion of water into wine at Cana. A third element is usually required, known as the commune tertium, which, even after conversion has taken place, either physically or at least logically unites one extreme to the other; for in every true conversion the following condition must be fulfilled: “What was formerly A, is now B.” A very important question suggests itself as to whether the definition should further postulate the previous non-existence of the last extreme, for it seems strange that an existing terminus a quo, A, should be converted into an already existing terminus ad quem, B. If the act of conversion is not to become a mere process of substitution, as in sleight-of-hand performances, the terminus ad quem must unquestionably in some manner newly exist, just as the terminus a quo must in some manner really cease to exist. Yet as the disappearance of the latter is not attributable to annihilation properly so called, so there is no need of postulating creation, strictly so called, to explain the former’s coming into existence. The idea of conversion is amply realized if the following condition is fulfilled, viz., that a thing which already existed in substance, acquires an altogether new and previously non-existing mode of being. Thus in the resurrection of the dead, the dust of the human bodies will be truly converted into the bodies of the risen by their previously existing souls, just as at death they had been truly converted into corpses by the departure of the souls. This much as regards the general notion of conversion. Transubstantiation, however, is not a conversion simply so called, but a substantial conversion (conversio substantialis), inasmuch as one thing is substantially or essentially converted into another. Thus from the concept of Transubstantiation is excluded every sort of merely accidental conversion, whether it be purely natural (e.g. the metamorphosis of insects) or supernatural (e.g. the Transfiguration of Christ on Mount Tabor). Finally, Transubstantiation differs from every other substantial conversion in this, that only the substance is converted into another — the accidents remaining the same — just as would be the case if wood were miraculously converted into iron, the substance of the iron remaining hidden under the external appearance of the wood. " imprimatur et al
newadvent.org/cathen/05572c.htm
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