Objection 1. It would seem that it was unfitting to the words, "Thou shalt love the Lord thy God, with thy whole heart," to add, "and with thy whole soul, and with thy whole strength" (Deuteronomy 6:5). For heart does not mean here a part of the body, since to love God is not a bodily action: and therefore heart is to be taken here in a spiritual sense. Now the heart understood spiritually is either the soul itself or part of the soul. Therefore it is superfluous to mention both heart and soul.
Objection 2. Further, a man's strength whether spiritual or corporal depends on the heart. Therefore after the words, "Thou shalt love the Lord thy God with thy whole heart," it was unnecessary to add, "with all thy strength."
Objection 3. Further, in Matthew 22:37 we read: "With all thy mind," which words do not occur here. Therefore it seems that this precept is unfittingly worded in Deuteronomy 6.
On the contrary stands the authority of Scripture.
I answer that, This precept is differently worded in various places: for, as we said in the first objection, in Deuteronomy 6 three points are mentioned: "with thy whole heart," and "with thy whole soul," and "with thy whole strength." On Matthew 22 we find two of these mentioned, viz. "with thy whole heart" and "with thy whole soul," while "with thy whole strength" is omitted, but "with thy whole mind" is added. Yet in Mark 12 we find all four, viz. "with thy whole heart," and "with thy whole soul," and "with thy whole mind," and "with thy whole force" which is the same as "strength." Moreover, these four are indicated in Luke 10, where in place of "strength" or "force" we read "with all thy might." [St. Thomas is explaining the Latin text which reads "ex tota fortitudine tua" (Deuteronomy), "ex tota virtue tua" (Mark), and "ex omnibus tuis" (Luke), although the Greek in all three cases has ex holes tes ischyos, which the Douay renders "with thy whole strength."]
Accordingly these four have to be explained, since the fact that one of them is omitted here or there is due to one implying another. We must therefore observe that love is an act of the will which is here denoted by the "heart," because just as the bodily heart is the principle of all the movements of the body, so too the will, especially as regards the intention of the last end which is the object of charity, is the principle of all the movements of the soul. Now there are three principles of action that are moved by the will, namely, the intellect which is signified by "the mind," the lower appetitive power, signified by "the soul"; and the exterior executive power signified by "strength," "force" or "might." Accordingly we are commanded to direct our whole intention to God, and this is signified by the words "with thy whole heart"; to submit our intellect to God, and this is expressed in the words "with thy whole mind"; to regulate our appetite according to God, in the words "with thy whole soul"; and to obey God in our external actions, and this is to love God with our whole "strength," "force" or "might."
Chrysostom [The quotation is from an anonymous author's unfinished work (Opus imperf. Hom. xlii, in Matth.) which is included in Chrysostom's works], on the other hand, takes "heart" and "soul" in the contrary sense; and Augustine (On Christian Doctrine I:21) refers "heart" to the thought, "soul" to the manner of life, and "mind" to the intellect. Again some explain "with thy whole heart" as denoting the intellect, "with thy whole soul" as signifying the will, "with thy mind" as pointing to the memory. And again, according to Gregory of Nyssa (De Hom. Opif. viii), "heart" signifies the vegetative soul, "soul" the sensitive, and "mind" the intellective soul, because our nourishment, sensation, and understanding ought all to be referred by us to God.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)