Resumen: This article will examine a total of twelve drawings which illustrate six of the 17
surviving letters from Peirce¿s first trip to Europe (June 1870- March 1871)2. The illustrations
are simple, but they are outstanding examples of one of the deepest convictions of Peirce:
Reason is not a mechanical skill and thought is not a linear process. A broader notion of
reason, that is, reasonableness, makes sense of Peirce's use of drawings and diagrams, since
one of the key elements of reasonableness is the imagination. According to Peirce, reasoning
is also a visual and diagrammatic process. In his letters Peirce includes drawings that illustrate
and clarify what he means.
Accordingly, the paper is arranged in three sections. First, we discuss some of the
results of our research into Peirce's European correspondence relating to art and aesthetics,
since they help to understand the context in which Peirce wrote the letters with the
illustrations. Second, we give a presentation of Peirce's notion of reason and his idea of visual
and diagrammatic thinking; finally, we include and briefly discuss a selection of the
illustrations found in Peirce's European letters from 1870-71.
