A great insight of Rudolf Steiner's, and one which should be of special relevance to all intellectuals; was that thinking is potentially a superb means of becoming aware of, understanding, and learning from the spiritual world beyond the five senses.
The comnon tendency is to suppose that spiritual knowing only comes from visions or hearing voices, from being overwhelmed by conviction, and in altered states of consciousness such as trances, lucid dreams or when in ill or intoxicated situations.
But Steiner (who perhaps generated more material, more words, on the subject of spiritual experience and knowledge than anybody else, ever) affirmed many times that thinking was not only possible but the best method of attaining spiritual knowledge.
Of course, we know that we can think about (almost) anything; and that is the basis of Steiner's insight. But the question is about the validity of this thinking.
There are many, obvious objections to Steiner's claim - and of course he does not mean that all thinking is always spiritually valid (that would be silly); rather he is claiming that clear, focused, purposive thinking, when directed towards certain subjects, and with certain motivations, is a source of spiritual knowledge - but he is always insistent that this aimed-at type of thinking has a form much more like the lucidity and alert-awakeness of scientific thinking; than it is like any state of 'possession', or anything dreamy or psychotic.
To do this; thinking should be embarked-upon in an undistracted situation where concentration and clarity are possible, and directed towards thinking about spiritual subjects of compelling (spontaneous) personal interest.
Once some purchase is obtained, some knowledge emerges as convincing; then thinking will move more and more into the spiritual world - and expand into the world of reality beyond the five senses.
In other words, we need not 'perceive' the spiritual world with 'senses' (as 'hallucinations') - rather, we can think the spiritual world: or, we perceive by thinking.
To get started on this requires no more than to become convinced that thinking (of this kind) may be a valid source of true knowledge; and to put it to the test of experience.
Note: Why should thinking be valid as a source of spiritual knowedge? The ultimate answer must be that it is built-into us, by God, for such a purpose. But it has not been much used in past ages as a source of super-sensory knowledge because this type of scientific thinking requires a 'modern' autonomous self - and this is why it is the destined primary spiritual method of the future.