The indestructibility of matter is also stated by Adelard’s contemporary, Hugh of St. Victor, who remarks in the _Didascalicon_ that of earthly things which have a beginning and an end “it has been said, ‘Nothing in the universe ever dies because no essence perishes.’ For the essences of things do not change, but the forms. And when a form is said to change, it should not be so understood that any existing thing is believed to perish utterly and lose its being, but only to undergo alteration, either perchance so that those things which were joined are separated, or those joined which had been separated....”[84] Hugh was quite certainly a younger man than Adelard, but it is not so certain that the _Didascalicon_ was written after the _Natural Questions_, although it is probable. Or Hugh may have heard Adelard lecture in Gaul or learned his view concerning the indestructibility of matter indirectly. Or they both may have drawn it independently from a common source.[85] [Sidenote: Roger Bacon’s continuity of universal nature.] In an article entitled _Roger Bacon et l’Horreur du Vide_[86] Professor Pierre Duhem advanced the thesis that in place of the previous doctrine that nature abhors a vacuum Roger Bacon was the first to formulate a theory of universal continuity. This was an incorrect hypothesis, it is true, but one which Professor Duhem believed to have served the useful purpose of supplementing “the Peripatetic theory of heavy and light” until the discovery of atmospheric pressure. This theory developed in connection with certain problematical phenomena of which this “experiment” is the chief and typical case. If there be suspended in air a vessel of water having a hole in the top and several narrow apertures in the bottom, no water will fall from it as long as the superior aperture is closed. Yet water is heavier than air and according to the principles of Aristotle’s _Physics_ should fall to the ground. Writers before Roger Bacon, according to Duhem, explain this anomaly by saying that the fall of the water would produce a vacuum and that a vacuum cannot exist in nature. But Bacon argues that a vacuum cannot be the reason why the water does not fall, because a vacuum does not exist; he then explains further that although by their particular natures water tends downwards and air upwards, by their nature as parts of the universe they tend to remain in continuity. Duhem held that Roger Bacon was the first to substitute this positive law of universal continuity for the mere negation that a vacuum cannot exist in nature.[87] [Sidenote: Previously stated by Adelard.]