Philosophers are sworn, aye, every one,
That they will thus discover it to none,
Nor in a book will write it for men here;
For unto Christ it is so lief and dear
That He wills that it not discovered be,
Except where it’s pleasing to his deity
Man to inspire, and also, to defend
Whom that he will; and lo, this is the end.

And thus do I conclude, since God in heaven
Wills that philosophers shall not say even
How any man may come upon that stone,
I say, as for the best, let it alone.
For whoso makes of God his adversary,
To work out anything that is contrary
To what he wills, he’ll surely never thrive.

The Canon’s Yeoman’s Tale:
From The Canterbury Tales (by Geoffrey Chaucer, c. 1343 – 25 October 1400).

Why is the recipe of the philosopher’s stone still a secret?