Argument from desire

The argument from desire is an argument for the existence of God used in Christian apologetics. It runs roughly as follows: This is an argument from analogy, claiming that because the desire for spiritual fulfillment is similar to desires for food and water in one way (we naturally desire them), they must be similar in another way (the object of that desire must really exist).
 * We have natural desires for certain things (ie. hunger, thirst).
 * Feeling these natural desires presupposes that certain things exist. (i.e., hunger presupposes the existence of food; thirst presupposes the existence of water) If those things did not exist, we wouldn’t naturally desire them.
 * We have a natural desire to be spiritually fulfilled.
 * This presupposes that certain things exist (ie. a soul, spiritual fulfillment, a.k.a. Heaven, a thing that can grant spiritual fulfillment, a.k.a. God).
 * Therefore those things exist.

Aquinas’ arguments
Thomas Aquinas used this argument to prove the existence of an immortal soul:

Aquinas also argued that spiritual satisfaction, which he called felicity, could not be achieved in this life. Nevertheless, he maintained that it could be achieved after this life because:

Natural desires versus artificial ones
The immediate problem is obvious: just because we want something doesn’t mean that thing exists. Defenders, however, point out that the argument only applies to natural or innate desires, not artificial ones. We naturally desire food and water, but we don’t naturally desire to fly or to become president. Rather, those desires are artificial — that is, they’re based on ideas shown or taught to us by society and the external world.

But it is debatable whether the desire for spiritual fulfillment is really a “natural desire” in the same sense as hunger and thirst or if this is a faulty analogy. After all, “(i)f the desires for food, water, sex and other basics are never fulfilled, the human race dies out. The “hunger” for the supernatural has nothing to do with survival…

The innate desires (of hunger and thirst) "are those we share with other social animals. Since no animal desires God, why call that desire innate?"

Similarly, "(t)he category of innate desires is those things for which there is a clear target of the desire. No one doubts that food and drink exist, but there’s plenty of doubt about superpowers." One would think/hope that if God created us with a need for Him, He would reveal Himself to us in more obvious ways.

=== Other Criticisms=== Underlying the argument is the idea that “nature does nothing in vain”, a teleological claim. In other words, if something is natural, it must have a purpose. This claim is dubious, as it’s possible for something to naturally exist with no greater purpose in mind (i.e., nihilism) or for something to naturally exist but no longer have a purpose (i.e., vestigial organs).

Which God and Heaven?
Even if Heaven and God do exist, this argument doesn’t prove which Heaven and God exist. It could conceivably be any Heaven or God (or heavens and gods) that humanity has dreamt up or even some that we haven’t dreamt up. Even supporters of the argument such as Peter Kreeft concede this point:

Can we reach them?
Even if the argument holds and the soul, God and Heaven all exist, that does not mean we can achieve them. Plenty of people want clean water but die of dehydration anyway. Plenty of people want food but die of starvation anyway. Likewise, even if Heaven and God exist and we naturally desire them, that doesn’t prove that we have the means to achieve them. Again, even supporters of the argument like C. S. Lewis admit this.

Likewise, even if we consider the desire for spiritual fulfillment to be like the desires for food and water, it’s possible that we will never be spiritually satisfied. After all, even if you eat to your heart’s (or stomach’s) content now, eventually you will become hungry again. Even if you drink to your heart’s (or liver’s) content now, eventually you will become thirsty again. In other words, even if we have a natural desire and we fulfill it, said fulfillment is only temporary and not permanent.