On the contrary, in its own being and nature, thinking belongs in the spiritual world. We already experience the spiritual world, though not consciously, when we really think; i.e. when our thinking is not merely acting as a mirror reflecting external phenomena.
When we are engaged in real thinking, then we have the possibility of experiencing ourselves as thinkers.
If man becomes conscious of himself within thinking, he knows himself to be in a world that exists beyond life and death.
Nothing is more certain than when man thinks he is then active as a spiritual being.
Edited from Lecture 1 of Rudolf Steiner's The Karma of Materialism - lectures given in 1917
I find the above passage to be striking and absolutely convincing. This is not a matter of evidence - indeed I can't imagine that any form of evidence could be adduced either for or against its truth. It is a metaphysical statement concerning our nature and the world - and as such we can/ must accept or reject it.
Either our real thinking is of this nature, or it is not - and the truth of the matter is only ascertained as a consequence of some kind of thinking.
For me, the above passage from Steiner resonates immediately with my own deepest intuitions - more than this, it describes my intuitions about intuitive thinking itself.
Much hinges on the distinction between real thinking and that which is merely a mirror of external phenomena - in its purest form this is a feature of early childhood. A young child is hardly conscious of himself thinking - experience flows into him, he is immersed-in phenomena, he is passive to experience; or else he acts, unthinkingly, from instinct.
As we develop, we separate our-selves from phenomena - eventually this separation is experienced as complete. Much (or all) of our thinking is still dictated by phenomena - especially in this days of mass and social media; and when not by phenomena, by memories of phenomena.
Perhaps very little thinking each day, perhaps none, is generated from our real-selves - uncaused thinking - thinking from that within us which is divine... but that is the real thinking to which Steiner refers. Only that thinking is 'in a world that exists beyond life and death'.
How do we know when it is happening? It is by an intuitive certainty of reflection: we think, we know we think, and we know that this thinking is real... That is what Steiner means when he says we 'know' ourselves in a world beyond life and death, and are 'certain' of this.
I understand this to mean that such a situation is as certain as anything can be to us - despite that modernity has trained us that such intuitions have zero intrinsic validity.
Steiner remarks elsewhere that when a man begins to doubt his own intuitions, he is in a terrible situation - he has in practice become a nihilist, doubting his own thinking he necessarily doubts everything - including the validity of that self which does the doubting...
And then He Is Lost - and passive to manipulation; which is the normal situation that has been engineered by The Establishment, and embraced by The Masses in this time and place.
But if you personally would instead prefer to experience living as a spiritual being in a world beyond life and death - the possibility is in your own hands: or thoughts. It is the primary act of the Romantic Christian.