Quoted from Karl Otto Apel’s Towards a Transformation of Philosophy (italics in original):
1. There can be no knowledge of something as something without a real sign mediation on the basis of material sign vehicles.
2. The sign can have no representational function for a consciousness without the presupposed existence of a real world, which in principle, must thought of as being representable, and that means knowable, in various respects.
3. There can be no representation of something as something by a sign without interpretation by a real interpreter.
Point #1 is the basis of structuralism in all its forms (original, neo, and post, etc.). Peirce is along with Saussure the father of semiotics/”sign”ology. Linguistics that studies sign as signs–grammar, usage, development, sentence formation–begins the Linguistic Shift in Philosophy characteristic of 20th century thought.
Point #2 is Peirce’s overcoming of the false Kantian noumena/phenomena divide–the dead end of that route of Kantian (and Neo-Kantian) philosophy. Peirce, as Apel shows, is walking a fine line. Peirce with the notion of a knowable/representable world is not going back to a pre-Kantian metaphysical correspondence view of truth whereby, we see things (to put it crudely) as they “really are”. At the same time he is “semioticizing” or rather pragmaticizing the divide between noumena and phenomena. Peirce describes what could be known and what we do know, but again the key point is by shifting to a linguistic mode as conversation (notice the reference to a consciousness not sign materialism only) the notion of a transcendental mind version via Kant is dissolved. There is still roughly a notion then of a transcendental foundation and the practical daily world, but it is one based on a community of understanding/interpretation not structures of the Universe. It is a knowable in “various respects” in other words.
Which leads directly to #3 and here is where Peirce (and Apel and Habermas) break with structuralism: that there is an interpreter. Structuralism proclaimed the End of Man and The End of Humanism and metaphysically prioritized structure over consciousness. In Derrida this shows up as grammar (grammatology) over logos.
The interpreter however involves (as it were) an interpretee–the consciousness Peirce speaks of in #2 is not an isolated monadic consciousness but a consciousness in discussion, in conversation. And this conversation then presumes the possibility of understanding by the other.
Point #3 is also a dagger at the heart of the philosophy of consciousness and neo-positivism (to use Habermas’ terms). That is, the assumption that what is the case can simply be described. The notion of Language as a Mirror or simple perfect 1:1 translation of what we experience (empiricism) or know (philosophy of consciousness).
And from here, Apel and Habermas, each in their own way, are going to argue for a post-metaphysics predicated around the description/investigation of how people communicate, the rules by which they discourse, the ways in which we intuitively presume to be able to understand each other, and universalize our answers. In the practicalities of how we deal in this world. Our actions, choices, our commitments.
If we believe that truth is found simply by scientific method–then there is no interpretation. But even a scientist must communicate to another scientist and assumes a language game (in Wittgenstein’s terms), a way of validity in a community right/wrong.