Let us for the moment name this substance _koilon_, since it fills what we are in the habit of calling empty space. What mûlaprakrti, or "mother-matter," is to the inconceivable totality of universes, koilon is to our particular universe--not to our solar system merely but to the vast unit which includes all visible suns. Between koilon and mûlaprakrti there must be various stages, but we have at present no direct means of estimating their number or of knowing anything whatever about them. In an ancient occult treatise, however, we read of a "colorless spiritual fluid" "which exists everywhere and forms the first foundation on which our solar system is built. Outside the latter, it is found in its pristine purity only between the stars [suns] of the universe.... As its substance is of a different kind from that known on earth, the inhabitants of the latter, seeing _through it_, believe, in their illusion and ignorance, that it is empty space. There is not one finger's breadth of void space in the whole boundless universe."[21] "The mother-substance" is said, in this treatise, to produce this æther of space as its seventh grade of density, and all objective suns are said to have this for their "substance." To any power of sight which we can bring to bear upon it, this koilon appears to be homogeneous, though it is probably nothing of the kind, since homogeneity can belong to the mother-substance alone. It is out of all proportion denser than any other substance known to us, infinitely denser--if we may be pardoned the expression; so much denser that it seems to belong to another type, or order, of density. But now comes the startling part of the investigation: we might expect matter to be a densification of this koilon; it is nothing of the kind. Matter is not koilon, but _the absence of koilon_, and at first sight, matter and space appear to have changed places, and emptiness has become solidity, solidity has become emptiness.