In its own way, Saya no Uta apparently adresses the inexistence of such a thing as absolute (classical) philosophical cathegories: truth, beauty and goodness. Specifically, and against naive realism, it shows that what our empirical culture takes as an esssential component of the ultimate path to objectivity, the notion of heavily considering the "object" as what is reported by our five senses, is flawed to the core. The story reminds us that every definition or provisional so called "truth" can only be grasped thanks to actual lies: to substitutions (by a semiotical process) and biased interpretations of stimuli, which ultimately render every human experience as virtual, fictional and thus essentially false. Then, these conclusions are made extensive to both our inherited concepts of beauty and goodness (i.e. that there are no aesthetical or moral truths, either). Nevertheless, Saya, for good or bad, ends up defending love (whatever that may be) as the ultimate value to cherish: the only thing worthy of truly sacrificing a world for; and we ourselves have to ackonowledge all this as well, in order to love and folow her to whatever may come...
Also: she's mighty brilliant, loverly perverse, heartbreaking (and melancholically) romantic and a super attractive petite sexual animal. Oh: and she's like a cat. And the game's art is damned stylish, the music's great and there's no waste on any route.