Peirce’s theory of signs is a rich and expansive theoretical option for cognitive sciences that does not assume the presence of the distinction between the methods of natural and those of the human sciences. The potential of the sign-theoretic account remains largely unacknowledged, however. The reason may be the conceptual jungle that one encounters at the moment one tries to penetrate it. The present paper explains the key terms of Peirce’s theory from the viewpoint of the theory of cognition. In Peirce’s own terms, this is to take the theory of signs to be influenced by phaneroscopy, the science of phenomenology that prepares ground for the sign-theoretic study of mind. We review the main technical terms of Peirce’s phaneroscopy and provide an explanation of its central nomenclature, often drawing from unpublished manuscript sources. This extended glossary can serve as an integrated aid to both Peirce’s theory of signs as well as to its phenomenological underpinnings, illustrating the unique character of this early method for the theory of cognition.