Although its particles are always in intensely rapid motion among themselves, this body has at the same time a kind of loose organization.
The size and shape of the mental body are determined by those of the causal vehicle.

There are in it certain striations which divide it more or less irregularly into segments, each of these corresponding to a certain department of the physical brain, so that every type of thought should function through its duly assigned portion.

The mental body is as yet so imperfectly developed in ordinary men that there are many in whom a great number of special departments are not yet in activity, and any attempt at thought belonging to those departments has to travel round through some inappropriate channel which happens to be fully open.

The result is that thought on those subjects is for those people clumsy and uncomprehending.
This is why some people have a head for mathematics and others are unable to add correctly--why some people instinctively understand, appreciate and enjoy music, while others do not know one tune from another.
All the matter of the mental body should be circulating freely, but sometimes a man allows his thought upon a certain subject to set and solidify, and then the circulation is impeded, and there is a congestion which presently hardens into a kind of wart on the mental body.

Such a wart appears to us down here as a prejudice; and until it is absorbed and free circulation restored, it is impossible for the man to think truly or to see clearly with regard to that particular department of his mind, as the congestion checks the free passage of undulations both outward and inward.
When a man uses any part of his mental body it not only vibrates for the time more rapidly, but it also temporarily swells out and increases in size.