RARELY has obedience laid upon me so difficult a task as this of writing about prayer; for one reason, because I do not feel that God has given me either the power or the desire for it, besides which, during the last three months I have suffered from noises and a great weakness in my head that have made it painful for me to write even on necessary business. 
However, as I know the power obedience has of making things easy which seem impossible, my will submits with a good grace, although nature seems greatly distressed, for God has not given me such strength as to bear, without repugnance, the constant struggle against illness while performing many different duties. May He, Who has helped me in other more difficult matters, aid me with His grace in this, for I trust in His mercy. I think I have but little to say that has not already been put forth in my other works written under obedience; in fact, I fear this will be but repetition of them. I am like a parrot which has learnt to talk; only knowing what it has been taught or has heard, it repeats the same thing over and over again. If God wishes me to write anything new, He will teach it me, or bring back to my memory what I have said elsewhere. I should be content even with this, for as I am very forgetful, I should be glad to be able to recall some of the matters about which people say I have spoken well, lest they should be altogether lost. If our Lord will not even grant me this, still, if I weary my brains and increase my headache by striving to obey, I shall gain in merit, though my words should be useless to any one. So I begin this work on the Feast of the Blessed Trinity in the year 1577, in the Convent of St. Joseph of Carmel at Toledo, where I am living, and I submit all my writings to the judgment of those learned men by whose commands I undertake them. That it will be the fault of ignorance, not malice, if I say anything contrary to the doctrine of the Holy Roman Catholic Church, may be held as certain. By God's goodness I am, and always shall be, faithful to the Church, as I have been in the past. May He be for ever blessed and glorified. Amen.
He who bids me write this, tells me that the nuns of these convents of our Lady of Carmel need some one to solve their difficulties about prayer: he thinks that women understand one another's language best and that my sisters' affection for me would make them pay special attention to my words, therefore it is important for me to explain the subject clearly to them. Thus I am writing only to my sisters; the idea that any one else could benefit by what I say would be absurd. Our Lord will be doing me a great favour if He enables me to help but one of the nuns to praise Him a little better; His Majesty knows well that I have no other aim. If anything is to the point, they will understand that it does not originate from me and there is no reason to attribute it to me, as with my scant understanding and skill I could write nothing of the sort, unless God, in His mercy, enabled me to do so.
 'I am always suffering from my head.' Letter of June 28, 1577. Letters. VOL. II.