Objection 1. It seems that madmen and imbeciles should not be baptized. For in order to receive Baptism, the person baptized must have the intention, as stated above (Article 7). But since madmen and imbeciles lack the use of reason, they can have but a disorderly intention. Therefore they should not be baptized.
Objection 2. Further, man excels irrational animals in that he has reason. But madmen and imbeciles lack the use of reason, indeed in some cases we do not expect them ever to have it, as we do in the case of children. It seems, therefore, that just as irrational animals are not baptized, so neither should madmen and imbeciles in those cases be baptized.
Objection 3. Further, the use of reason is suspended in madmen and imbeciles more than it is in one who sleeps. But it is not customary to baptize people while they sleep. Therefore it should not be given to madmen and imbeciles.
On the contrary, Augustine says (Confess. iv) of his friend that "he was baptized when his recovery was despaired of": and yet Baptism was efficacious with him. Therefore Baptism should sometimes be given to those who lack the use of reason.
I answer that, In the matter of madmen and imbeciles a distinction is to be made. For some are so from birth, and have no lucid intervals, and show no signs of the use of reason. And with regard to these it seems that we should come to the same decision as with regard to children who are baptized in the Faith of the Church, as stated above (9, ad 2).
But there are others who have fallen from a state of sanity into a state of insanity. And with regard to these we must be guided by their wishes as expressed by them when sane: so that, if then they manifested a desire to receive Baptism, it should be given to them when in a state of madness or imbecility, even though then they refuse. If, on the other hand, while sane they showed no desire to receive Baptism, they must not be baptized.
Again, there are some who, though mad or imbecile from birth, have, nevertheless, lucid intervals, in which they can make right use of reason. Wherefore, if then they express a desire for Baptism, they can be baptized though they be actually in a state of madness. And in this case the sacrament should be bestowed on them if there be fear of danger otherwise it is better to wait until the time when they are sane, so that they may receive the sacrament more devoutly. But if during the interval of lucidity they manifest no desire to receive Baptism, they should not be baptized while in a state of insanity.
Lastly there are others who, though not altogether sane, yet can use their reason so far as to think about their salvation, and understand the power of the sacrament. And these are to be treated the same as those who are sane, and who are baptized if they be willing, but not against their will.
Reply to Objection 1. Imbeciles who never had, and have not now, the use of reason, are baptized, according to the Church's intention, just as according to the Church's ritual, they believe and repent; as we have stated above of children (9, ad Objection). But those who have had the use of reason at some time, or have now, are baptized according to their own intention, which they have now, or had when they were sane.
Reply to Objection 2. Madmen and imbeciles lack the use of reason accidentally, i.e. through some impediment in a bodily organ; but not like irrational animals through want of a rational soul. Consequently the comparison does not hold.
Reply to Objection 3. A person should not be baptized while asleep, except he be threatened with the danger of death. In which case he should be baptized, if previously he has manifested a desire to receive Baptism, as we have stated in reference to imbeciles: thus Augustine relates of his friend that "he was baptized while unconscious," because he was in danger of death (Confess. iv).
Source: Summa Theologica (New Advent)