The differences between the first and second stages are: (a) in the first there is no effort to understand the subject or object upon which attention rests whereas in the second there is; (b) concentration may be directed to any physical thing or mental idea whereas meditation must be directed to thinking about a spiritual theme either logically or imaginatively.
In the third stage this theme pervades the mind so completely that the thinking activity ceases, the thoughts and fancies vanish. The meditator and his theme are then united; it is no longer separate from him. Both merge into a single consciousness. To shut off all perceptions of the outer world, all physical sense-activities of seeing hearing and touching, is the goal and end of the first stage. It is achieved when concentration on one subject or object is fully achieved. To shut off all movements of the inner world, all mental activities of thinking, reasoning, and imagining, is the goal and end of the second stage. It is achieved when the subject or object pervades awareness so completely that the meditator forgets himself and thus forgets even to think about it: he is it. To shut off all thoughts and things, even all sense of a separate personal existence, and rest in contemplation of the One Infinite Life-Power out of which he has emerged, is the goal and end of the third stage.
-- Notebooks Category 4: Elementary Meditation > Chapter 1: Preparatory > # 214