The Argument from Desire
Thomas Aquinas, writing on this, appeals to an Aristotelian principle:
"It is impossible for natural desire to be unfulfilled, since nature does nothing in vain. Now, natural desire would be in vain if it could never be fulfilled. Therefore, man's natural desire is capable of fulfillment, but not in this life...So, it must be fulfilled after this life. Therefore, man's ultimate felicity comes after this life."1
If we put the argument in a syllogism it would look like this:
1. Natural desires are such that they are not in vain and have an object capable of fulfilling them.
2. Man experiences immaterial desires.
3. Therefore, there is something transcendent to matter that can fulfill immaterial desires.
Catholic philosopher Peter Kreeft puts it this way:
1.Every natural, innate desire in us corresponds to some real object that can satisfy that desire.
2. But there exists in us a desire which nothing in time, nothing on earth, no creature can satisfy.
3. Therefore there must exist something more than time, earth, and creatures, which can satisfy the desire. 2
Suffice it to say our natural desire for that elusive "something", which is never satisfied, but only briefly quieted by material objects, does indeed point to the necessity for something beyond this material world.
In evangelism efforts I wouldn't use the Argument from Desire as a stand alone argument, since it will rely on other more substantive arguments, but it is a good argument to use with those who cite feelings as evidence of some truth they wish to establish. While feelings are not a trustworthy source of input in the quest for truth, desires certainly can be.
1. Thomas Aquinas, "On the Truth of the Catholic Faith", Book 3, Part 1 (1945) Hanover House. p. 166
2. Peter Kreeft, "C.S. Lewis' Argument from Desire", in "The Riddle of Joy: G.K. Chesterton and C.S. Lewis" (1989) Eerdmans.