In Peircean terminology, Turning Signs could be described as a hybrid of cenoscopy and synthetic philosophy. Cenoscopy, as opposed to the idioscopic or specialized sciences such as physics and psychology, investigates
phenomena that are perfectly familiar to all mankind. Because these are founded on common observation, Bentham gave them the collective designation Cenoscopy, which I adopt as expressive of my own opinion of the basis on which these sciences, which are otherwise called Philosophy, rest.— Peirce, MS 601 (c. 1906)
Cenoscopy then ‘embraces all that positive science which rests upon familiar experience and does not search out occult or rare phenomena’; for Peirce this, rather than metaphysics, is the real “first philosophy,” or at least ‘is better entitled (except by usage) to being distinguished as philosophia prima than ontology’ (EP2:372). Synthetic philosophy, on the other hand, ‘has been called philosophia ultima’ because it ‘embraces all that truth which is derivable by collating the results of different special sciences, but which is too broad to be established by any one of them’ (EP2:372).
In other words, the philosophical inquiry reflected in Turning Signs aims at both the primary (or primal?) and the ultimate – the alpha and the omega. This makes it doubly useful in these apocalyptic or transitional times. Actually only the cenoscopic part should be called “inquiry,” or heuretic science as Peirce called it. He placed synthetic philosophy ‘at the head of the Retrospective Sciences’ (EP2:373), i.e. those which find new connections among observations previously made rather than making new observations of their own. But the reliance of cenoscopic inquiry on ‘familiar experience’ does not make it easier to practice, because it requires critical common sense.
The method of cenoscopic research presents a certain difficulty. In commencing it we are confronted with the fact that we already believe a great many things. These beliefs, or at least the more general of them, ought to be reconsidered with deliberation. This implies that it should be conducted according to a deliberate plan adopted only after the severest criticism. Indeed, nothing in cenoscopy should be embraced without criticism. Each criticism should wait to be planned, and each plan should wait for criticism. Clearly, if we are to get on at all, we must put up with imperfect procedure.— Peirce, EP2:373
This is roughly equivalent to Merleau-Ponty’s observation about phenomenology: ‘The most important lesson of the reduction is the impossibility of a complete reduction.’