[...] What you say about the third act of Tristan frees you from me, while on the other hand the pain it has cost you to say it, shows me that we must yet do something together. Do not blame your physical powers if you are afraid of being unequal to the exertions of this third act. Let the very nature of a theatrical performance convince you of the true view, namely, that where the profoundest subtleties must be illustrated by immediate action, there reigns a different logic than that which governs a purely poetic conception. It is thinkable and possible that this third act of Tristan may some day be given in its entirety; but this is impracticable for a first performance. [...] Even if the singer were able to achieve it perfectly forthwith, such prolixity would be more than hazardous at a first performance before the theatrical public. [...]
Therefore, do not let your noble zeal for the task grow cool. Sooner or later you will sing Tristan, and then the cuts I have found necessary—on many considerations—will make the part as practicable and easy as you could wish.
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 506