From Self-Development and the Way to Power, by L. W. Rogers (6)

Our evolution is going forward and has gone forward in the past.
We know that the human race has passed through a long evolution during
which it has acquired five senses by which knowledge is gained. Nobody
who has given thought to the subject will make the mistake of
supposing that this evolution is completed and that the five senses
are all we shall ever possess.

In this long evolutionary journey the next thing we shall do is to
develop the sixth sense. Some people have already done so and all are
approaching it. This dawning sense is called clairvoyance. Fair
investigation will show that the clairvoyant possesses certain powers
not common to the majority of people. This is merely the beginning of
the development of the sixth sense, and probably with the majority of
clairvoyants it goes no further than etheric and lower astral sight.
In other words, they are able to raise the consciousness only to a
grade of matter a little beyond the grasp of ordinary vision, while
the properly developed, trained clairvoyant raises his consciousness
two full planes beyond.

The higher the consciousness is raised the further the horizon of
knowledge extends and the clairvoyant is able to hand down information
that appears quite miraculous; but it is perfectly natural. If a
certain person were born blind and had never understood any more about
eyesight than most people understand about clairvoyance; if this
person could know how many doorways were in a large building only by
groping along with his hands and thus acquiring the knowledge by
touch, and another person who could see should glance along the block
and instantly tell the blind man the correct number, that would be to
the blind man a miracle. Now, when a clairvoyant sees things at a
distance where the physical eye cannot reach he really does nothing
more remarkable. When we see a thing we receive the vibrations caused
by light. That gives the information. When the clairvoyant "sees" at a
distance through what we mistakenly call solid substances he receives
vibrations of matter so fine that it interpenetrates solids as the
ether does.