Objection 1. It seems that a sacrament is not always something sensible. Because, according to the Philosopher (Prior. Anal. ii), every effect is a sign of its cause. But just as there are some sensible effects, so are there some intelligible effects; thus science is the effect of a demonstration. Therefore not every sign is sensible. Now all that is required for a sacrament is something that is a sign of some sacred thing, inasmuch as thereby man is sanctified, as stated above (Article 2). Therefore something sensible is not required for a sacrament.
Objection 2. Further, sacraments belong to the kingdom of God and the Divine worship. But sensible things do not seem to belong to the Divine worship: for we are told (John 4:24) that "God is a spirit; and they that adore Him, must adore Him in spirit and in truth"; and (Romans 14:17) that "the kingdom of God is not meat and drink." Therefore sensible things are not required for the sacraments.
Objection 3. Further. Augustine says (De Lib. Arb. ii) that "sensible things are goods of least account, since without them man can live aright." But the sacraments are necessary for man's salvation, as we shall show farther on (61, 1): so that man cannot live aright without them. Therefore sensible things are not required for the sacraments.
On the contrary, Augustine says (Tract. lxxx super Joan.): "The word is added to the element and this becomes a sacrament"; and he is speaking there of water which is a sensible element. Therefore sensible things are required for the sacraments.
I answer that, Divine wisdom provides for each thing according to its mode; hence it is written (Wisdom 8:1) that "she... ordereth all things sweetly": wherefore also we are told (Matthew 25:15) that she "gave to everyone according to his proper ability." Now it is part of man's nature to acquire knowledge of the intelligible from the sensible. But a sign is that by means of which one attains to the knowledge of something else. Consequently, since the sacred things which are signified by the sacraments, are the spiritual and intelligible goods by means of which man is sanctified, it follows that the sacramental signs consist in sensible things: just as in the Divine Scriptures spiritual things are set before us under the guise of things sensible. And hence it is that sensible things are required for the sacraments; as Dionysius also proves in his book on the heavenly hierarchy (Coel. Hier. i).
Reply to Objection 1. The name and definition of a thing is taken principally from that which belongs to a thing primarily and essentially: and not from that which belongs to it through something else. Now a sensible effect being the primary and direct object of man's knowledge (since all our knowledge springs from the senses) by its very nature leads to the knowledge of something else: whereas intelligible effects are not such as to be able to lead us to the knowledge of something else, except in so far as they are manifested by some other thing, i.e. by certain sensibles. It is for this reason that the name sign is given primarily and principally to things which are offered to the senses; hence Augustine says (De Doctr. Christ. ii) that a sign "is that which conveys something else to the mind, besides the species which it impresses on the senses." But intelligible effects do not partake of the nature of a sign except in so far as they are pointed out by certain signs. And in this way, too, certain things which are not sensible are termed sacraments as it were, in so far as they are signified by certain sensible things, of which we shall treat further on (63, 1, ad 2; 3, ad 2; 73, 6; 74, 1, ad 3).
Reply to Objection 2. Sensible things considered in their own nature do not belong to the worship or kingdom of God: but considered only as signs of spiritual things in which the kingdom of God consists.
Reply to Objection 3. Augustine speaks there of sensible things, considered in their nature; but not as employed to signify spiritual things, which are the highest goods.
Source: Summa Theologica (New Advent)