Before we begin the study of different types of meditation it is necessary to keep in mind two important points. One is that meditation is not just ordinary concentration but a special type of internal concentration.
Knowledge originates in two ways. One is direct perception in which the senses receive energy from the external world. The other is memory, which is the result of the sprouting of samskaras or latent impressions of past experience lying buried in the mind. Just as a tape-recorder when played back reproduces the original sounds, so also latent impressions in the mind when activated recreate the original experience.
Meditation is the bridge that connects the lower mind with the higher mind. Through that the aspirant crosses over from the din and distractions of the sense-bound world to the world of stillness and silence, from the world of darkness to the world of everlasting light. All preliminary spiritual disciplines end in meditative awareness.
When you sit for meditation and close your eyes, almost the first thing you notice is that your awareness is not continuous. It does not consist of a single, homogenous stream but flows as different, sometimes disconnected, streams of thought. Psychologists call this phenomenon "dissociation." By dissociation is meant not the appearance of various pictures in the mind, but the emotional sectioning of the mind and the identification of the self with each division.
Dhyana or meditation is the conscious maintenance of a steady stream of the same thought about an object at a higher center of consciousness.1 What we call thinking is the manipulation of a series of thought-waves called vrittis.