(The name astral was given to it by mediaeval alchemists, because its matter is starry or shining as compared to that of the denser world.) The seventh world, composed of the type of matter which we see all around us, is called the physical.
The matter of which all these interpenetrating worlds are built is essentially the same matter, but differently arranged and of different degrees of density.

Therefore the rates at which these various types of matter normally vibrate differ also.

They may be considered as a vast gamut of undulations consisting of many octaves.

The physical matter uses a certain number of the lowest of these octaves, the astral matter another group of octaves just above that, the mental matter a still further group, and so on.
Not only has each of these worlds its own type of matter; it has also its own set of aggregations of that matter--its own substances.

In each world we arrange these substances in seven classes according to the rate at which their molecules vibrate.