From Herr B.'s letter you will see that he is honest enough to admit that he does not understand the meaning of the paticcasamuppāda formula, which he rightly describes as 'difficult'. At the same time he has observed that kāya-, vacī-, and citta-sankhāra cannot be identified with kāya-, vacī-, and mano-sañcetanā, and he consequently approves what I have written about these terms in the Notes. You may remember that this was one of the points about which I wrote to you at some length (about December 1963). Anyway, here is independent confirmation (if you need it) that my view that these two sets of terms must be kept distinct (they are confused in the Visuddhimagga) is not without foundation. Herr B. is right to want to make clear the distinction between citta, mano, and viññāna, but his ideas about citta are a little mixed up. (Actually, these words, and especially citta, have variable meanings according to their context—like 'mind' and 'consciousness' in English, and the task Herr B. has set himself—to write a thesis on these three terms—is more difficult than he supposes.)
The word samsāra comes from sam plus sarati; sarati means 'to go, flow, run, move', etc. and sam is an intensifying prefix. Samsarati therefore means literally 'to go on, to flow on, to run on, or to move on'; and there is nothing in the word samsāra itself to justify its translation as 'cycle or round of rebirths'. And also, as you say, we do not traverse the same existence twice. Actually, this book, Mindfulness of Breathing, is an early translation of the Ven. Ñānamoli Thera's (possibly he might not have approved its being reprinted), and his later translation of samsāra is simply 'round-about'. Though there is no etymological justification for such a rendering, it perhaps conveys something of the endless repetition of 'birth, ageing, and death', and then back to 'birth' again. We do not, certainly, repeat any given birth, ageing or death; but we do repeat the cycle of birth, ageing and death. No doubt the translation of samsāra as 'cycle of rebirths' has been encouraged by the (erroneous) view that the paticcasamuppāda formulation represents a cycle of three successive existences—indeed, the twelve terms of the p.s. are sometimes represented in the form of a circle (see, for example, the Ven. Piyadassi Thera's booklet 'Dependent Origination', Wheel 15). As far as I remember, I used to translate samsāra as 'the course' or 'the coursing on' (on referring to my new glossary in the Notes, I see that I have written: samsāra—running on [from existence to existence]).
We are very short of rain in this district, and no cultivation has been possible in this season. I have enough water for drinking and sponging down of the body, but I shall have to do without proper baths.