I answer that, The eye is of two kinds, namely, the bodily eye properly so-called, and the intellectual eye, so-called by similitude. But Christ's body as it is in this sacrament cannot be seen by any bodily eye. First of all, because a body which is visible brings about an alteration in the medium, through its accidents. Now the accidents of Christ's body are in this sacrament by means of the substance; so that the accidents of Christ's body have no immediate relationship either to this sacrament or to adjacent bodies; consequently they do not act on the medium so as to be seen by any corporeal eye. Secondly, because, as stated above (A[1], ad 3; A[3]), Christ's body is substantially present in this sacrament. But substance, as such, is not visible to the bodily eye, nor does it come under any one of the senses, nor under the imagination, but solely under the intellect, whose object is "what a thing is" (De Anima iii). And therefore, properly speaking, Christ's body, according to the mode of being which it has in this sacrament, is perceptible neither by the sense nor by the imagination, but only by the intellect, which is called the spiritual eye.
Moreover it is perceived differently by different intellects. For since the way in which Christ is in this sacrament is entirely supernatural, it is visible in itself to a supernatural, i.e. the Divine, intellect, and consequently to a beatified intellect, of angel or of man, which, through the participated glory of the Divine intellect, sees all supernatural things in the vision of the Divine Essence. But it can be seen by a wayfarer through faith alone, like other supernatural things. And not even the angelic intellect of its own natural power is capable of beholding it; consequently the devils cannot by their intellect perceive Christ in this sacrament, except through faith, to which they do not pay willing assent; yet they are convinced of it from the evidence of signs, according to James 2:19 : "The devils believe, and tremble."
Reply to Objection 1: Our bodily eye, on account of the sacramental species, is hindered from beholding the body of Christ underlying them, not merely as by way of veil (just as we are hindered from seeing what is covered with any corporeal veil), but also because Christ's body bears a relation to the medium surrounding this sacrament, not through its own accidents, but through the sacramental species.
Reply to Objection 2: Christ's own bodily eye sees Himself existing under the sacrament, yet it cannot see the way in which it exists under the sacrament, because that belongs to the intellect. But it is not the same with any other glorified eye, because Christ's eye is under this sacrament, in which no other glorified eye is conformed to it.
Reply to Objection 3: No angel, good or bad, can see anything with a bodily eye, but only with the mental eye. Hence there is no parallel reason, as is evident from what was said above.
Whether Christ's body is truly there when flesh or a child appears miraculously in this sacrament?
Objection 1: It seems that Christ's body is not truly there when flesh or a child appears miraculously in this sacrament. Because His body ceases to be under this sacrament when the sacramental species cease to be present, as stated above (A[6]). But when flesh or a child appears, the sacramental species cease to be present. Therefore Christ's body is not truly there.
Objection 2: Further, wherever Christ's body is, it is there either under its own species, or under those of the sacrament. But when such apparitions occur, it is evident that Christ is not present under His own species, because the entire Christ is contained in this sacrament, and He remains entire under the form in which He ascended to heaven: yet what appears miraculously in this sacrament is sometimes seen as a small particle of flesh, or at times as a small child. Now it is evident that He is not there under the sacramental species, which is that of bread or wine. Consequently, it seems that Christ's body is not there in any way.
Objection 3: Further, Christ's body begins to be in this sacrament by consecration and conversion, as was said above (Q[75], AA[2],3,4). But the flesh and blood which appear by miracle are not consecrated, nor are they converted into Christ's true body and blood. Therefore the body or the blood of Christ is not under those species.
On the contrary, When such apparition takes place, the same reverence is shown to it as was shown at first, which would not be done if Christ were not truly there, to Whom we show reverence of "latria." Therefore, when such apparition occurs, Christ is under the sacrament.
I answer that, Such apparition comes about in two ways, when occasionally in this sacrament flesh, or blood, or a child, is seen. Sometimes it happens on the part of the beholders, whose eyes are so affected as if they outwardly saw flesh, or blood, or a child, while no change takes place in the sacrament. And this seems to happen when to one person it is seen under the species of flesh or of a child, while to others it is seen as before under the species of bread; or when to the same individual it appears for an hour under the appearance of flesh or a child, and afterwards under the appearance of bread. Nor is there any deception there, as occurs in the feats of magicians, because such species is divinely formed in the eye in order to represent some truth, namely, for the purpose of showing that Christ's body is truly under this sacrament; just as Christ without deception appeared to the disciples who were going to Emmaus. For Augustine says (De Qq. Evang. ii) that "when our pretense is referred to some significance, it is not a lie, but a figure of the truth." And since in this way no change is made in the sacrament, it is manifest that, when such apparition occurs, Christ does not cease to be under this sacrament.
But it sometimes happens that such apparition comes about not merely by a change wrought in the beholders, but by an appearance which really exists outwardly. And this indeed is seen to happen when it is beheld by everyone under such an appearance, and it remains so not for an hour, but for a considerable time; and, in this case some think that it is the proper species of Christ's body. Nor does it matter that sometimes Christ's entire body is not seen there, but part of His flesh, or else that it is not seen in youthful guise. but in the semblance of a child, because it lies within the power of a glorified body for it to be seen by a non-glorified eye either entirely or in part, and under its own semblance or in strange guise, as will be said later (XP, Q[85], AA[2],3).
But this seems unlikely. First of all, because Christ's body under its proper species can be seen only in one place, wherein it is definitively contained. Hence since it is seen in its proper species, and is adored in heaven, it is not seen under its proper species in this sacrament. Secondly, because a glorified body, which appears at will, disappears when it wills after the apparition; thus it is related ( Lk. 24:31 ) that our Lord "vanished out of sight" of the disciples. But that which appears under the likeness of flesh in this sacrament, continues for a long time; indeed, one reads of its being sometimes enclosed, and, by order of many bishops, preserved in a pyx, which it would be wicked to think of Christ under His proper semblance.
Consequently, it remains to be said, that, while the dimensions remain the same as before, there is a miraculous change wrought in the other accidents, such as shape, color, and the rest, so that flesh, or blood, or a child, is seen. And, as was said already, this is not deception, because it is done "to represent the truth," namely, to show by this miraculous apparition that Christ's body and blood are truly in this sacrament. And thus it is clear that as the dimensions remain, which are the foundation of the other accidents, as we shall see later on (Q[77], A[2]), the body of Christ truly remains in this sacrament.
Reply to Objection 1: When such apparition takes place, the sacramental species sometimes continue entire in themselves; and sometimes only as to that which is principal, as was said above.
Reply to Objection 2: As stated above, during such apparitions Christ's proper semblance is not seen, but a species miraculously formed either in the eyes of the beholders, or in the sacramental dimensions themselves, as was said above.
Reply to Objection 3: The dimensions of the consecrated bread and wine continue, while a miraculous change is wrought in the other accidents, as stated above.