It seems to me that the first reason that Jesus called this man a fool was because he allowed the “within” of his life to be absorbed within the “without” of his life. Each of us lives on two levels, so to speak, and we operate within two realms—the “within” and the “without.” The “within” of life is what we use. It deals with the techniques and devices, instrumentalities and mechanisms, by means of which we live—in short, the material stuff that is necessary for our existence. This is the “without” of life—the car we drive, the house we live in, the clothes we wear, and all of those material objects that are necessary for our earthly survival. Then there is a “within” of life. And this is that realm of spiritual ends, which expresses itself in art, literature, morality and religion, for which, at best, we live.
Now, the foolishness of this man consisted in the fact that he allowed the “within" of his life to become absorbed in the “without.” In other words, he allowed the means by which he lived to become, to absorb the ends for which he lived.
There's a lot more in the sermon, which I think is one of his best, and I think is as relevant today as it ever has been: the problem of people letting means absorb ends has not grown less.