Answer

I don't have much to say about it, other than that each of us nevertheless has the sense that we do in fact have tools to understand the idea from the whole. You could even put this to an empirical test: take a text by someone living today and interpret it, and then check with him how far off we were. I assume there would be misses, but it would not be completely disconnected, as the deconstructionists (Derrida) claim.
Gadamer and others juggle words in order to try to ground this intuitive thesis (fusion of horizons, etc.), but I have found nothing useful at all in what they say. Bottom line: it is an empirical fact that we have such an ability, and that's that. In principle, one could ask how we know that our senses reflect reality, since everything we know about it comes through the senses. That too is a hermeneutic circle, and there too, anyone who is not a skeptic or a philosophical idealist nevertheless trusts the senses. There too there is no good philosophical answer to the matter, but the fact is that we believe the senses, and that it works.a0