Peirce’s philosophical views had idealist underpinnings. He had four systems. In his first system (1859–1861), he agreed with Immanuel Kant (1724–1804) that things-in-themselves could not be known either in science or philosophy. Science is concerned with phenomena, or what appears in experience. But there is an objective world underlying phenomena, or what is known. There are three kinds of things: 1) matter; 2) mind; and 3) God, or “It,” “Thou,” and “I,” which Peirce called “Firstness,” “Secondness” and “Thirdness,” respectively. Peirce thought that ideas in God’s mind are as material as objects in our experience. However, he encountered logical problems with this system and was not quite satisfied with the relation between the Kantian categories and the things in themselves.

In his second system of thought (1866–1970), Peirce used Hegelian methodology and assumptions to conclude that what was most real was a dynamic system. He thought that the world of experience or phenomena, which he called “the phaneron,” is entirely made up of signs which are qualities, relations, things, events—everything—and that these signs are all meaningful. The meaning of each sign is part of a system that also contains the object and the “interpretant.” The object is what the sign is a sign of. The interpretant is the feature or activity of mind that experiences the sign. And, the interpretant is also a sign—because everything is a sign—so it also has an object and a second interpretant.

This structure of sign—object—interpretant, interpretant-as-sign → object → new interpretant goes on infinitely. But the reality of the object consists of a limiting form that is approached as cognitions approach infinity. That is, if an object is real, our process of inquiry and experience can go on almost forever. Reality for Peirce was a “convergence of inquiry,” and since what we know is always general or a universal, the object is made up of universals. This makes reality mental, hence Peirce’s philosophical idealism.

However, Peirce ran into difficulties with the logic of these relations, and after discovering an original (and still not widely understood, except by logicians) logic of relations, he constructed his third system (1870–1884), which more closely resembled what is now considered pragmatism and is based on the operating principles that most now associate with Peirce, although he called his system “pragmaticism” to distinguish it from the ideas of other pragmatists, who were less concerned with science.