The letter is too long to be given in full, but a few quotations may suffice to indicate its importance to those who desire to comprehend the man.
"You must know then that from my earliest infancy I found always a strong inclination to books and letters.

As our college education in Scotland, extending little further than the languages, ends commonly when we are about fourteen or fifteen years of age, I was after that left to my own choice in my reading, and found it incline me almost equally to books of reasoning and philosophy, and to poetry and the polite authors.

Every one who is acquainted either with the philosophers or critics, knows that there is nothing yet established in either of these two sciences, and that they contain little more than endless disputes, even in the most fundamental articles.

Upon examination of these, I found a certain boldness of temper growing on me, which was not inclined to submit to any authority in these subjects, but led me to seek out some new medium, by which truth might be established.

After much study and reflection on this, at last, when I was about eighteen years of age, there seemed to be opened up to me a new scene of thought, which transported me beyond measure, and made me, with an ardour natural to young men, throw up every other pleasure or business to apply entirely to it.