In response to this letter from Tranquillina, the following letter is delivered by Renaud late in the spring of 1224. It is written upon papyrus, sealed in wax that contains a piece of walnut shell still embedded in the seal.
Fair greetings, sodalis. Please forgive this aging old man who could not directly remember receiving the letter to which you referred; fortunately it was still ensconced in my stored belongings, and hence I was able after reflection to call our introduction to mind. You have moved quite a great distance since that time, as you say - perhaps not so unusual, although I far prefer to remain here amidst the stability of home and family. Your prayers on their behalf are kind and welcome: when we received your last letter, Semira called for a mass to amplify your intentions and to echo the Lord's favor back upon you. My children and grandchildren are well indeed, although my nephew Sopros was thrown from his horse and is confined to his Convalescence Bed; I thank you for your prayers in his name also. Arbus Tutus (my 7th) is now raising a daughter, praise God, and will soon begin siring sons of my great-great-grandfather's bloodline.
But you asked about Hermetic matters, in particular the art of diagnosis. The best use of this quill is to inform you that much of what I might tell you is also available to the Order at large, as I have sent two volumes detailing my research to your House's vaunted library. Most in demand will be my treatise on Longevity Rituals and fertility, but your inquiry leads me to suggest the other volume, in which I describe Artemis' Fertility which demonstrably increases the likelihood of conception for the spell's beneficiary. Among other incantations, the diagnostic The Maculate Noblewoman is most directly related to your question; I should attempt to summarize its main ideas briefly here, for your medical beliefs (if I might say so without offending) are suffering from a few minor misapprehensions.
First, while you are correct that the humours derive from the chyle that is delivered to the liver via the appetitive force, it is not the liver alone that mediates the second concoction producing the four humours, but only the aspect governing the sanguineous humour. Indeed the natural faculty must be distributed as far as the pituitary before all four humours can be derived. On the plus side, this demonstrates that your hypothetical Intellego spell can be centered upon one of these organs alone, the pituitary in your example of detecting the creation of the serous humour. Contrarily, Intellego trained upon the liver alone cannot detect the balance of humours. Moreover, the transformation into cambion (and, more pertinently, sperma) is far removed from this second concoction, and so - while the balance of the humours is of course essential for the health of the human vessel in which flourishes the cambion, it is not directly involved in that creation. In fact my grandfather Alboin was the first to demonstrate conclusively that cambion can indeed be magically created....
(He includes a brief description of his grandfather's spell, then goes into more details about his diagnostic incantation and the elements of Corpus magic that differentiate its effects on the various bodily tissues and fluids. Much of this description is understandable, but a few details involving both advanced medical knowledge and newly discovered Hermetic theory are difficult for even Tranquillina to fully comprehend.)
In this way, we see that additionally to the reduction or amplification of any one humour, it is crucial that the total fluid content of the patient be preserved, else one might induce hypohydration or, in the other direction, even burst an organ with excess. This is the danger (one danger - not the only) of weaving Rego power into a diagnostic spell, which is why I prefer to regulate the patient with mundane methods (simple fluid intake, administering a spontaneous purgatory, or leeching) and confine such investigations to the Technique of Intellego alone.
Perhaps these simple observations might assist you in pulling together a lesser version of The Maculate Noblewoman, to allow some investigation into your servant's condition while you gain access to the perfected spell at Durenmar; I hope I have given you some reason to be hopeful. I will pray for her fertility at the next full moon. Your invitation is most kind, but I have a large family to oversee here, and indeed enchantments to design for the first of my great-grandchildren.
Respectfully,
Aurulentus ex Jerbiton