"I wish to describe to you how powerful is my desire to rejoice with you and to cause you to rejoice at your simchah. I do not exaggerate when I tell you how many years I have waited and longed for this moment and how many times I have pictured in my mind's eye how I would dance in front of you at your wedding. But, "Many are the thoughts of men..." (Mishlei 19:21). This too is no doubt also for the good. But know, my dear friend, that only my three-dimensional body, bound by the contraints of time and space, will not be with you. My soul, unconstrained by distance and above all time and space, the true "I," which is joined to you with all of its might, will have a large share in your simchah."
- Reb Shraga Feivel Mendelovitz, 1886-1948 (quoted in Moshe Bamberger, Great Jewish Letters, pg. 203)
"Our holy Torah places emphasis upon knowing and understanding the numerous worlds that exist, including this world and the World to Come, and the concepts of life and death. Once we believe in the absolute Oneness of G-d and of His name - which is the very essence of our Torah and our sacred faith - then there cannot be any division whatsoever between the worlds, between this world and the Coming World, and between life and death...
"I have recently seen the comments of the holy Ohr ha'Chaim, who explains that death should be understood as simply moving from one city to another (Leviticus 26:3). This is the whole truth. Your father, may his memory be blessed, has not died; he is alive. He has merely moved. To the understanding person there is even more to say. The deceased is now even closer to you than before, for now there are no separations. He is alive! He is now nearby and with us at all times. We must think in these terms and remember him, lest we forget him by considering him to be gone - G-d forbid."
- Reb Yerucham Levovitz, 1874-1936 (quoted in Moshe Bamberger, Great Jewish Letters, pg. 260)
See Derashot ha'Ran, Derush Shvi'i; Shiure Da'at, Chayei Olamot, and Saul-Paul Sirag's Consciousness and Hyperspace.