The garden is as central to the concept of an Islamic home as the hearth is to the European home. It is interesting, then, that the hearth-fire in old traditions has a similar association with the life of the inhabitants of the house.
Commonly, the fire of the hearth was not allowed to go out. It was carefully covered with ashes each night at curfew so that a few selected embers would survive until morning. (In fact, the word "curfew" originated from the French word for cover-the-fire—couvre-feu.) Raglan comments that "the alarm and horror felt if the hearth-fire went out are out of all proportion to the inconvenience caused" by the need to relight it.
Thermal information is not differentiated in our memory; rather it is retained as a quality, or underlying tone, associated with the whole experience of the place. It contributes to our sense of the particular personality, or spirit, that we identify with that place. In remembering the spirit of a place, we can anticipate that if we return, we will have the same sense of comfort or relaxation as before.
Now I sometimes wonder whether the current of utility has not become too strong and whether there would be sufficient opportunity for a full life if the world were emptied of some of the useless things that give it spiritual significance; in other words, whether our conception of what is useful may not have become too narrow to be adequate to the roaming and capricious possibilities of the human spirit.
Wang tiles (Hao Wang, 1961) are a class of formal systems. They are modelled visually by square tiles with a color on each side. A set of such tiles is selected, and copies of the tiles are arranged side by side with matching colors, without rotating or reflecting them.
The basic question about a set of Wang tiles is whether it can tile the plane or not, i.e., whether an entire infinite plane can be filled this way. The next question is whether this can be done in a periodic pattern.
In 1966, Wang's student Robert Berger solved the problem in the negative. He proved that no algorithm for the problem can exist, by showing how to translate any Turing machine into a set of Wang tiles that tiles the plane if and only if the Turing machine does not halt. The undecidability of the halting problem then implies the undecidability of Wang's tiling problem.