Your questions cover the whole of a very wide field. It is therefore necessary to reply to them with some brevity, touching only on some principal points.Sri Aurobindo
Letters of Sri Aurobindo
06. Sadhana through Meditation
Fragment ID: 1367
Your questions cover the whole of a very wide field. It is therefore necessary to reply to them with some brevity, touching only on some principal points.
1. What meditation exactly means.
There are two words used in English to express the Indian idea of dhyāna, “meditation” and “contemplation”. Meditation means properly the concentration of the mind on a single train of ideas which work out a single subject. Contemplation means regarding mentally a single object, image, idea so that the knowledge about the object, image or idea may arise naturally in the mind by force of the concentration. Both these things are forms of dhyāna, for the principle of dhyāna is mental concentration whether in thought, vision or knowledge.
There are other forms of dhyāna. There is a passage in which Vivekananda advises you to stand back from your thoughts, let them occur in your mind as they will and simply observe them and see what they are. This may be called concentration in self-observation.
This form leads to another, the emptying of all thought out of the mind so as to leave it a sort of pure vigilant blank on which the divine knowledge may come and imprint itself, undisturbed by the inferior thoughts of the ordinary human mind and with the clearness of a writing in white chalk on a blackboard. You will find that the Gita speaks of this rejection of all mental thought as one of the methods of yoga and even the method it seems to prefer. This may be called the dhyāna of liberation, as it frees the mind from slavery to the mechanical process of thinking and allows it to think or not to think, as it pleases and when it pleases, or to choose its own thoughts or else to go beyond thought to the pure perception of Truth called in our philosophy Vijñāna.
Meditation is the easiest process for the human mind, but the narrowest in its results; contemplation more difficult, but greater; self-observation and liberation from the chains of Thought the most difficult of all, but the widest and greatest in its fruits. One can choose any of them according to one’s bent and capacity. The perfect method is to use them all, each in its own place and for its own object; but this would need a fixed faith and firm patience and a great energy of Will in the self-application to the yoga.
2. What should be the object or ideas for meditation?
Whatever is most consonant with your nature and highest aspirations. But if you ask me for an absolute answer, then I must say that Brahman is always the best object for meditation or contemplation and the idea on which the mind should fix is that of God in all, all in God and all as God. It does not matter essentially whether it is the Impersonal or the Personal God, or subjectively, the One Self. But this is the idea I have found the best, because it is the highest and embraces all other truths, whether truths of this world or of the other worlds or beyond all phenomenal existence,– “All this is the Brahman”.
In the third issue of Arya, at the end of the second instalment of the Analysis of the Isha Upanishad, you will find a description of this vision of the All1 which may be of help to you in understanding the idea.
3. Conditions internal and external that are most essential for meditation.
There are no essential external conditions, but solitude and seclusion at the time of meditation as well as stillness of the body are helpful, sometimes almost necessary to the beginner. But one should not be bound by external conditions. Once the habit of meditation is formed, it should be made possible to do it in all circumstances, lying, sitting, walking, alone, in company, in silence or in the midst of noise etc.
The first internal condition necessary is concentration of the will against the obstacles to meditation, i.e. wandering of the mind, forgetfulness, sleep, physical and nervous impatience and restlessness etc.
The second is an increasing purity and calm of the inner consciousness (citta) out of which thought and emotion arise, 
i.e. a freedom from all disturbing reactions, such as anger, grief, depression, anxiety about worldly happenings etc. Mental perfection and moral are always closely allied to each other.
1 See Sri Aurobindo, Isha Upanishad (1965 Edition), p. 35.