4 After this the Priest cries aloud, “Lift up your hearts.” For truly ought we in that most awful hour to have our heart on high with God, and not below, thinking of earth and earthly things. In effect therefore the Priest bids all in that hour to dismiss all cares of this life, or household anxieties, and to have their heart in heaven with the merciful God. Then ye answer, “We lift them up unto the Lord:” assenting to it, by your avowal. But let no one come here, who could say with his mouth, “We lift up our hearts unto the Lord,” but in his thoughts have his mind concerned with the cares of this life. At all times, rather, God should be in our memory but if this is impossible by reason of human infirmity, in that hour above all this should be our earnest endeavour.
Source: Catechetical Lectures (New Advent)