One crux of the idealist teaching of Sri Ramana in particular, of Advaita Vedant in general can be found clearly elucidated by Michael James, who for many years was a disciple of Sri Sadhu Om (who was a disciple of Sri Ramana):

All thoughts are known by us only through the medium of our mind, which is our first thought ‘I’, but our essential consciousness ‘I am’ is known by us directly, not through our mind or any other medium. All the knowledge that we have of everything other than ‘I am’ depends for its seeming reality upon the reality of the mind through which we know it. If our mind is unreal, all things known by it must also be unreal, since they are only thoughts that it has formed within itself.

 Happiness and the Art of Being: An Introduction to the Philosophy and Practice of the Spiritual Teachings of Bhagavan Sri Ramana (2012; second edition), p. 172.

1. The mind rises as the “root” or “primal” thought I, says Sri Ramana. (Also translated as “I-thought” or “I-notion.”)

2. After the rising of the thought I, the mind creates, or projects, the body idea and world idea. (I mean after here in a logical, not in a temporal sense, since I-body-world arises concomitantly, so far as I can tell.)

3. How could it be that the mind could be unreal? In at least two senses, this is true. First, the mind is not in the driver’s seat; it is in the passenger’s seat while falsely presuming otherwise. Second, the mind has nothing but a borrowed existence, one that comes to it from the Self, and thus cannot stand on its own. It is not sat or Being.

4. The crux, then, is to be grasped here: “If our mind is unreal, all things known by it must also be unreal, since they are only thoughts that it has formed within itself.” There’s no sense in investigating simply the unreality of the world or the body since this can go on forever. Far better, and more skillful, to investigate their root–which is the thought “I,” or mind. Investigating the mind reveals that it does not exist, i.e., is unreal. Investigating that unreality reveals instead and all so plainly the pure actuality of “I am.” This is Pure Awareness.

5. What is elegant about Ramana’s version of the nondual teaching is that, especially in the hands of the humble and reflective Michael James, it gives us a skeptical method that drives toward Truth. The failure of Descartes is evident not so much in his availing himself of a certain skeptical method as it is his inability to take Subjectivity all the way to the end. The foundation of Subjectivity is not thought. Quite apperceptively, it is “I am,” “I-I,” “I am I,” or simply, equivalently “I,” an immediately evident fullness that is prior to and beyond all thought.

6. Abide as “I am I.” Establish one’s Understanding as This here, now.