Aquinas on transubstantion

"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. . . . And therefore, properly speaking, Christ's body, according to the mode of being which it has in the sacrament, is perceptible neither by sense nor by the imagination, but only by the intellect, which is called the spiritual eye."

"the accidents which are discerned by the senses are truly present.  But the intellect, whose proper object is substance . . . is preserved by faith from deception . . . because faith is not contrary to the senses, but concerns things which sense does not reach."

St. Thomas Aquinas, Summa theologiae III.76.7, as quoted by Reinhard Hütter, in his "Eucharistic adoration in the personal presence of Christ:  making explicit the mystery of faith by way of metaphysical contemplation," Nova et vetera:  the English edition of the international journal 7, no. 1 (Winter 2009):  208.