Peirce delineates his triad of image, diagram, and metaphor in the following passage.
Hypoicons may be roughly divided according to the mode of Firstness of which they partake. Those which partake of simple qualities, or First Firstnesses, are images; those which represent the relations, mainly dyadic, or so regarded, of the parts of one thing by analogous relations in their own parts, are diagrams; those which represent the representative character of a representamen by representing a parallelism in something else, are metaphors. [CP 2.277]
And where he goes on in other places to fill out what he meant by images and diagrams, this is pretty much it for metaphors. They run in parallel, a somewhat mystifying metaphor in its own right.
But Giambattista Vico says something interesting about metaphors that might help.
[120] By its nature, the human mind is indeterminate; hence, when man is sunk in ignorance, he makes himself the measure of the universe. [New Science, trans. by David Marsh, Element 1]
In other words, we make a metaphor of one system we know, the "vehicle" of our own selves, with another system, the "tenor" in question. The tenor is seen as working in "parallel" with how things work within us. Although the vehicle is not always ourselves, I think it's fair to say, with Vico, that the initial approach to any unknown thing is metaphorical correlation with something we do know.
But Vico goes on to say, looking at these metaphors marbled into culture as "poetic truth":
[205] … Indeed, if we consider the question carefully, poetic truth is metaphysical truth; and any physical truth which does not conform to it must be judged false. [Ibid., 47]
Human knowledge first turns to metaphor, to poetic truth, and then, only later, fills in the systemic details of the tenor diagrammatically. What's more, according to Vico, that physical truth must "conform to" the poetic truth. This seems questionable in the sense that any metaphorical understanding would surely be undone by different applications and delineations of rendering it diagrammatically. But then the metaphorical understanding could be expected to evolve as well. Perhaps it is a tandem unfolding of concepts in this way that Blumenberg is taking advantage of when he traces the historical evolution of concepts of like "progress" or "truth" metaphorically.
Hans Blumenberg, in "On a Lineage of the Idea of Progress" traces the notion of progress in astronomy back to Hipparchus and his star catalog:
… said to have listed 1,022 stars according to position and brightness, [that] was laid down as an instrument for future comparisons. [p. 9]
This revolutionary notion introduced a "coordinative relation between the quantum of time and the quality of achievement" [p. 6], the idea of postponing or deferring present assertions of knowledge until the necessary data and accuracy could be accumulated and realized in the future.
Copernicus, according to Blumenberg, built on Hipparchus' data but looked at where he fit into this notion of progress differently.
For all the time it takes, the theory of the universe struck him as a finite task ending with him. [p. 18]
And:
While recognizing himself as a beneficiary of the time that had passed since Ptolemy, he gives no sign of sharing the ancient astronomers' sense of a vast future time as a continuing astronomical need. [p. 19]
If this seems a bit hubristic, it is at this point where astronomy, and the sciences more generally, took off. Bruno, following Copernicus, seems to have looked at progress the same way.
Copernicus must not be judged by things whose accomplishment was beyond him; after all, he had been no more than the dawn that precedes the sunrise of true philosophy – by which Bruno meant none other than his own. [p. 24]
The evolution of science seems to have really gotten underway as a Hegelian progression where each new conceptualization, after correcting flaws in previous ones, takes itself as the "absolute," final, consistent and complete encompassing of the matter.
With Galileo, however, this hubris came to be publicly tempered. As Blumenberg puts it:
We must not state that the Copernican doctrine is true, but for this very reason we may continue our intensive research into the objects it refers to. Progress is merely the by-product of this enforced uncertainty, and of the corresponding effort in which we refuse to admit to ourselves that it is as futile as it is declared to be. [p. 26]
This recalls Peirce's insistence on fallibility, and seems common enough today, but what I find interesting is that, despite the professed uncertainty, "we refuse to admit to ourselves that it is as futile as it is declared to be." If concepts are to evolve, perhaps it is essential that each, in its heart of hearts, see itself as absolute.
