Objection 1. It seems that the body of Christ, as it is in this sacrament, can be seen by the eye, at least by a glorified one. For our eyes are hindered from beholding Christ's body in this sacrament, on account of the sacramental species veiling it. But the glorified eye cannot be hindered by anything from seeing bodies as they are. Therefore, the glorified eye can see Christ's body as it is in this sacrament.
Objection 2. Further, the glorified bodies of the saints will be "made like to the body" of Christ's "glory," according to Philippians 3:21. But Christ's eye beholds Himself as He is in this sacrament. Therefore, for the same reason, every other glorified eye can see Him.
Objection 3. Further, in the resurrection the saints will be equal to the angels, according to Luke 20:36. But the angels see the body of Christ as it is in this sacrament, for even the devils are found to pay reverence thereto, and to fear it. Therefore, for like reason, the glorified eye can see Christ as He is in this sacrament.
On the contrary, As long as a thing remains the same, it cannot at the same time be seen by the same eye under diverse species. But the glorified eye sees Christ always, as He is in His own species, according to Isaiah 33:17: "(His eyes) shall see the king in his beauty." It seems, then, that it does not see Christ, as He is under the species of this sacrament.
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 (1, ad 3; 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.
Source: Summa Theologica (New Advent)