The person who, without being clairvoyant, understands everything spiritual science has to say, is an initiate. However, he who can enter these worlds, which we call the invisible world, is a clairvoyant. In earlier times, not so long ago, there existed a strict separation in the secret schools, between clairvoyants and initiates. As an initiate, without being clairvoyant, a person could attain to knowledge of higher worlds, if only he or she used their intellect in the right way.
On the other hand, someone could be clairvoyant without being initiated into a particularly high degree. It will be clear to you what is meant. Imagine two people, a very scholarly man, who knows all that physics and physiology possibly have to say about light and the phenomena of light, but who is so near-sighted, that he can barely see ten centimetres in front of himself: his eyesight is weak, but he is thoroughly acquainted with the laws of light.
Similarly, someone can be initiated into the spiritual world, but still be unable to perceive clearly what he sees. Another has excellent sight in the outer sensory world but has almost no knowledge of everything the scholarly man knows. Thus, there can also be clairvoyants who are able to perceive the spiritual world – they see into the spiritual world, but have no realisation, no knowledge of that which they see.
Source (German): Rudolf Steiner – GA 56 – Die Erkenntnis der Seele und des Geistes – Berlin, October 10, 1907 (page 26)
Translated by Nesta Carsten-Krüger