Objection 1. It would seem that three fruits are unfittingly assigned to the three parts of continence: because twelve fruits of the Spirit are assigned, "charity, joy, peace," etc. (Galatians 5:22). Therefore seemingly we should reckon only three.
Objection 2. Further, fruit denotes a special reward. Now the reward assigned to virgins, widows, and married persons is not a special reward, because all who are to be saved are comprised under one of these three, since no one is saved who lacks continence, and continence is adequately divided by these three. Therefore three fruits are unfittingly assigned to the three aforesaid.
Objection 3. Further, just as widowhood surpasses conjugal continence, so does virginity surpass widowhood. But the excess of sixtyfold over thirtyfold is not as the excess of a hundredfold over sixtyfold; neither in arithmetical proportion, since sixty exceeds thirty by thirty, and a hundred exceeds sixty by forty; nor in geometrical proportion, since sixty is twice thirty and a hundred surpasses sixty as containing the whole and two-thirds thereof. Therefore the fruits are unfittingly adapted to the degrees of continence.
Objection 4. Further, the statements contained in Holy Writ stand for all time: "Heaven and earth shall pass away, but My words shall not pass away" (Luke 21:33): whereas human institutions are liable to change every day. Therefore human institutions are not to be taken as a criterion of the statements of Holy Writ: and it would seem in consequence that the explanation of these fruits given by Bede is unfitting. For he says (Expos. in Luc. iii, 8) that "the thirtyfold fruit is assigned to married persons, because in the signs drawn on the 'abacus' the number 30 is denoted by the thumb and index finger touching one another at the tips as though kissing one another: so that the number 30 denotes the embraces of married persons. The number 60 is denoted by the contact of the index finger above the middle joint of the thumb, so that the index finger by lying over the thumb and weighing on it, signifies the burden which widows have to bear in this world. When, however, in the course of enumeration we come to the number 100 we pass from the left to the right hand, so that the number 100 denotes virginity, which has a share in the angelic excellence; for the angels are on the right hand, i.e. in glory, while we are on the left on account of the imperfection of the present life."
I answer that, By continence, to which the fruit corresponds, man is brought to a kind of spiritual nature, by withdrawing from carnal things. Consequently various fruits are distinguished according to the various manners of the spirituality resulting from continence. Now there is a certain spirituality which is necessary, and one which is superabundant. The spirituality that is necessary consists in the rectitude of the spirit not being disturbed by the pleasures of the flesh: and this obtains when one makes use of carnal pleasures according to the order of right reason. This is the spirituality of married persons. Spirituality is superabundant when a man withdraws himself entirely from those carnal pleasures which stifle the spirit. This may be done in two ways: either in respect of all time past, present, and future, and this is the spirituality of virgins; or in respect of a particular time, and this is the spirituality of widows. Accordingly to those who keep conjugal continence, the thirtyfold fruit is awarded; to those who keep the continence of widows, the sixtyfold fruit; and to those who keep virginal continence, the hundredfold fruit: and this for the reason given by Bede quoted above, although another motive may be found in the very nature of the numbers. For 30 is the product of 3 multiplied by 10. Now 3 is the number of everything, as stated in De Coelo et Mundo i, and contains a certain perfection common to all, namely of beginning, middle, and end. Wherefore the number 30 is fittingly assigned to married persons, in whom no other perfection is added to the observance of the Decalogue, signified by the number 10, than the common perfection without which there is no salvation. The number six the multiplication of which by 10 amounts to 60 has perfection from its parts, being the aggregate of all its parts taken together; wherefore it corresponds fittingly to widowhood, wherein we find perfect withdrawal from carnal pleasures as to all its circumstances (which are the parts so to speak of a virtuous act), since widowhood uses no carnal pleasures in connection with any person, place, or any other circumstance; which was not the case with conjugal continence. The number 100 corresponds fittingly to virginity; because the number 10 of which 100 is a multiple is the limit of numbers: and in like manner virginity occupies the limit of spirituality, since no further spirituality can be added to it. The number 100 also being a square number has perfection from its figure: for a square figure is perfect through being equal on all sides, since all its sides are equal: wherefore it is adapted to virginity wherein incorruption is found equally as to all times.
Reply to Objection 1. Fruit is not taken there in the sense in which we are taking it now.
Reply to Objection 2. Nothing obliges us to hold that fruit is a reward that is not common to all who will be saved. For not only the essential reward is common to all, but also a certain accidental reward, such as joy in those works without which one cannot be saved. Yet it may be said that the fruits are not becoming to all who will be saved, as is evidently the case with those who repent in the end after leading an incontinent life, for to such no fruit is due but only the essential reward.
Reply to Objection 3. The distinction of the fruits is to be taken according to the species and figures of the numbers rather than according to their quantity. Nevertheless even if we regard the excess in point of quantity, we may find an explanation. For the married man abstains only from one that is not his, the widow from both hers and not hers, so that in the latter case we find the notion of double, just as 60 is the double of 30. Again 100 is 60 X 40, which latter number is the product of 4 X 10, and the number 4 is the first solid and square number. Thus the addition of this number is fitting to virginity, which adds perpetual incorruption to the perfection of widowhood.
Reply to Objection 4. Although these numerical signs are a human institution, they are founded somewhat on the nature of things, in so far as the numbers are denoted in gradation, according to the order of the aforesaid joints and contacts.
Source: Summa Theologica (New Advent)