Objection 1. It would seem that corporal almsdeeds have not a spiritual effect. For no effect exceeds its cause. But spiritual goods exceed corporal goods. Therefore corporal almsdeeds have no spiritual effect.
Objection 2. Further, the sin of simony consists in giving the corporal for the spiritual, and it is to be utterly avoided. Therefore one ought not to give alms in order to receive a spiritual effect.
Objection 3. Further, to multiply the cause is to multiply the effect. If therefore corporal almsdeeds cause a spiritual effect, the greater the alms, the greater the spiritual profit, which is contrary to what we read (Luke 21:3) of the widow who cast two brass mites into the treasury, and in Our Lord's own words "cast in more than... all." Therefore bodily almsdeeds have no spiritual effect.
On the contrary, It is written (Sirach 17:18): "The alms of a man... shall preserve the grace of a man as the apple of the eye."
I answer that, Corporal almsdeeds may be considered in three ways. First, with regard to their substance, and in this way they have merely a corporal effect, inasmuch as they supply our neighbor's corporal needs. Secondly, they may be considered with regard to their cause, in so far as a man gives a corporal alms out of love for God and his neighbor, and in this respect they bring forth a spiritual fruit, according to Sirach 29:13-14: "Lose thy money for thy brother... place thy treasure in the commandments of the Most High, and it shall bring thee more profit than gold."
Thirdly, with regard to the effect, and in this way again, they have a spiritual fruit, inasmuch as our neighbor, who is succored by a corporal alms, is moved to pray for his benefactor; wherefore the above text goes on (Sirach 29:15): "Shut up alms in the heart of the poor, and it shall obtain help for thee from all evil."
Reply to Objection 1. This argument considers corporal almsdeeds as to their substance.
Reply to Objection 2. He who gives an alms does rot intend to buy a spiritual thing with a corporal thing, for he knows that spiritual things infinitely surpass corporal things, but he intends to merit a spiritual fruit through the love of charity.
Reply to Objection 3. The widow who gave less in quantity, gave more in proportion; and thus we gather that the fervor of her charity, whence corporal almsdeeds derive their spiritual efficacy, was greater.
Source: Summa Theologica (New Advent)