Reply Obj. 5: The blessing of the water is not essential to Baptism, but belongs to a certain solemnity, whereby the devotion of the faithful is aroused, and the cunning of the devil hindered from impeding the baptismal effect. _______________________
FOURTH ARTICLE [III, Q. 66, Art. 4]
Whether Plain Water Is Necessary for Baptism?
Objection 1: It seems that plain water is not necessary for Baptism. For the water which we have is not plain water; as appears especially in sea-water, in which there is a considerable proportion of the earthly element, as the Philosopher shows (Meteor. ii). Yet this water may be used for Baptism. Therefore plain and pure water is not necessary for Baptism.
Obj. 2: Further, in the solemn celebration of Baptism, chrism is poured into the water. But this seems to take away the purity and plainness of the water. Therefore pure and plain water is not necessary for Baptism.
Obj. 3: Further, the water that flowed from the side of Christ hanging on the cross was a figure of Baptism, as stated above (A. 3, ad 3). But that water, seemingly, was not pure, because the elements do not exist actually in a mixed body, such as Christ's. Therefore it seems that pure or plain water is not necessary for Baptism.
Obj. 4: Further, lye does not seem to be pure water, for it has the properties of heating and drying, which are contrary to those of water. Nevertheless it seems that lye can be used for Baptism; for the water of the Baths can be so used, which has filtered through a sulphurous vein, just as lye percolates through ashes. Therefore it seems that plain water is not necessary for Baptism.
Obj. 5: Further, rose-water is distilled from roses, just as chemical waters are distilled from certain bodies. But seemingly, such like waters may be used in Baptism; just as rain-water, which is distilled from vapors. Since, therefore, such waters are not pure and plain water, it seems that pure and plain water is not necessary for Baptism.
On the contrary, The proper matter of Baptism is water, as stated above (A. 3). But plain water alone has the nature of water. Therefore pure plain water is necessary for Baptism.
I answer that, Water may cease to be pure or plain water in two ways: first, by being mixed with another body; secondly, by alteration. And each of these may happen in a twofold manner; artificially and naturally. Now art fails in the operation of nature: because nature gives the substantial form, which art cannot give; for whatever form is given by art is accidental; except perchance when art applies a proper agent to its proper matter, as fire to a combustible; in which manner animals are produced from certain things by way of putrefaction.