2 In considering your letters, in answering all of which I have certainly had to answer questions of no small difficulty and importance, I was not a little stunned by the one in which you ask me by what means certain thoughts and dreams are put into our minds by higher powers or by superhuman agents. The question is a great one, and, as your own prudence must convince you, would require, in order to its being satisfactorily answered, not a mere letter, but a full oral discussion or a whole treatise. I shall try, however, knowing as I do your talents, to throw out a few germs of thought which may shed light on this question, in order that you may either complete the exhaustive treatment of the subject by your own efforts, or at least not despair of the possibility of this important matter being investigated with satisfactory results.
Source: Letters (New Advent)