In your recent letter, brother and dearest friend, you were anxious to persuade me to gather together certain rules of the art of logic into one treatise, and to send them to Your Honor. Since, therefore, moved by a love for your progress and for the truth, I cannot go against your requests, I shall try what you ask, and shall undertake a matter that is difficult for me but fruitful, I think, both for you and for me.

For logic is the most useful tool of all the arts. Without it no science can be fully known. It is not worn out by repeated use, after the manner of material tools, but rather admits of continual growth through the diligent exercise of any other science. For just as a mechanic who lacks a complete knowledge of his tool gains a fuller [knowledge] by using it, so one who is educated in the firm principles of logic, while he painstakingly devotes his labor to the other sciences, acquires at the same time a greater skill at this art.

William Ockham, Summa Logicae Prefatory Letter.