In Italian.

Pieri writes in praise of The Principles of Mathematics. 

His Italian is translated as follows in Elena Anne Marchisotto and James T. Smith's The Legacy of Mario Pieri in Geometry and Arithmetic (Boston: Birkhäuser, 2007), p. 386:

Most distinguished Doctor
Catania (Sicily), 22 May 1903

I thank you heartily for the splendid gift of the Principles of Mathematics; and even more for the favorable opinions you have expressed about some of my works. The principal assumption of your book conforms perfectly with my viewpoint. I, too, have always believed that the primitive objects of pure mathematics can all be defined by means of some logical categories (Cl, ε, ∋ (such that), etc. ...); in short, that the undefinable can be eliminated from all deductive sciences except from Logic; and that the primitive objects of that [science] are not subject to different interpretations; and therefore that they must rightly be called logical constants. I have indeed affirmed all these [opinions], although very timidly, somewhere in my works (as probable truths), and mention again that Professor [Giuseppe] Peano may now disagree with me on this matter. I shall read the book with great interest.

To you the most obliged
Mario Pieri