"… [D]eduction consists in constructing an icon or diagram the relations of whose parts shall present a complete analogy with those of the parts of the object of reasoning, of experimenting upon this image in the imagination, and of observing the result so as to discover unnoticed and hidden relations among the parts." [CP 3.363]However, while Peirce is specifically describing "deduction" as it functions within a certain kind of diagram here, deduction can also be understood syllogistically as it functions in the mediating aspect of diagrams more generally. For example:
All men are mortalThe major premise — "All men are mortal" — is part of an understanding of human beings, a diagram consisting of a set of beliefs regarding humans. The minor premise — "Socrates is a man" — is a factual statement applying that diagram to this aspect of experience, and the conclusion — "Socrates is mortal" — is a consequence, an expectation, deduced from the diagram. In this sense, the deductive aspect of a diagram lies in applying it to experience and using it, as the "middle term", to infer what can then be expected from experience.
Socrates is a man
Therefore, Socrates is mortal.