I tend to agree, though I suppose it's not entirely proven that it's never failed. But the "unthinakble thoughts" argument seems to be to go in two different di

I tend to agree, though I suppose it's not entirely proven that it's never failed. But the "unthinakble thoughts" argument seems to be to go in two different directions - the analogy to sound is misleading; we have reason to believe in the existence of ultrasonic vibrations, and they don't require us to hear or not hear them. But thoughts don't exist separate from our minds. Truly unthinkable thoughts wouldn't be thoughts at all. What the concept seems to represent here is "novel ways of thinking" and those are in fact generally not impossible to share with other thinkers (luckily).