William Quan Judge described concentration this way:
Real Concentration is in fact Union with the Divine. We are to understand that we are each the Divine. There is no separateness but the one Spirit is in each reflected in each person. This truth, expressed by the ancients as "Thou art that spirit" is to be well understood and felt before concentration can become possible. Ordinary concentration of attention is merely an outward show but of course necessary also in the real concentration. Now having deeply thought over this you should study such a book as Patanjali's yoga Philosophy, which is the philosophy of concentration and in which you should find much light on this topic. The true source for concentration is selflessness, for as long as we feel the shackles of the personal self, so long is concentration hindered in various ways.
- William Quan Judge letter to Mrs. Baber Pathorne. January 6, 1891. Published in Practical Occultism: From the Private Letters of William Q. Judge edited by Arthur L. Conger. Theosophical University Press, 1951. Available online at TUP website.