The Oriental sages make their disciples live for years a life of resignation and asceticism before they impart to them their own wisdom. The Western world no longer demands pious exercises and ascetic practices as a preparation for science, but it does require a sincere willingness to withdraw oneself awhile from the immediate impressions of life, and to betake oneself into the realm of pure thought. POF Original Preface
In the past one prepared to receive knowledge through pious exercises and ascetic practices, today one trains by practicing pure thinking. This is seen in the tough curriculum of the gurus of our modern age, the engineers and computer scientists. The barrier that blocks the unworthy is calculus. These graduates are the ones shaping our world with new knowledge as a result of intensive thought training in the pure thinking of mathematics and analytical mechanics. The next step for these highly disciplined minds is to expand this pure thinking into ethics (see Part II of The Philosophy Of Freedom).
A modern innovator in art or social fields is often also a kind of philosopher who practices pure thinking in philosophical reflection. Writers may take time each day to enter the realm of pure thought gaining the reputation as a “deep” thinker.
What is the realm of pure thought? A pure thought is a pure concept so we are talking about entering the conceptual sphere of pure concepts. There are pure concepts and individualized concepts. A pure concept has no content obtained from sense perception.
When a concept is combined with perceptible content is becomes an individualized concept or mental picture. For example:
My concept of a lion is not constructed out of my perceptions of a lion; but my mental picture of a lion is formed under the guidance of the perception. I can teach someone to form the concept of a lion without his ever having seen a lion, but I can never give him a living mental picture of it without the help of his own perception. POF 6-2
An individualized concept can be a memory picture, formed by looking at a triangle following in thought its outline, size, directions of its sides, etc., and then producing a conceptual photograph of it for myself, my mental picture of that particular triangle. A mental picture can also be formed by imaginatively adding perceptual content to a concept. A pure concept is different. My pure concept of the triangle is a single one, comprising every single triangle. My pure concept of a lion contains the qualities common to all lions.
If you would like to train your thinking by entering the realm of pure thought read a philosophy book, study math or theosophy; anything filled with pure concepts. I would highly recommend Rudolf Steiner's Philosophy Of Freedom. After a reading session of this book you will be able to say you had a good work out.