You should read Nietzsche, Wittgenstein and Levy Strauss then, and after that exploring the authors of the cultural and linguistic turn. But long story short, this task was indeed taken on by some people, and their conclusion is that the language and the discourse (which is symbols and conventions) is what inform the truth and a "blank state truth" doesn't exist. In your lone islander example, every conclusion the islander can come up with is related to the language he develops on the island to articulate his feelings.